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Recent debates on the development of detachment faults during continental extension have been centered on whether they were active at low angles and initiated as relatively planar faults. In addition, intense investigations have concentrated on how the detachment-fault geometry evolved in time and space and how the extension was accommodated in their upper and lower plates. The superb exposure of the mid-Tertiary Whipple Mountains detachment fault system in southeastern California provides an unusual opportunity to address these questions. Analysis of the stratigraphy and structures of upper-plate rock units constrains the three-dimensional kinematic evolution of the mid-Tertiary Whipple-Chemehuevi detachment fault system in southeastern California. Tertiary volcanic strata in the northwestern Whipple Mountains and central Chemehuevi Valley consist of similar rock types that have undergone about 15% extensional strain. In contrast, a different succession of Tertiary volcanic and sedimentary rocks underlies the northeastern Whipple Mountains along the structural trend of central Chemehuevi Valley; these strata have been extended by 40%. Differences in magnitudes of extension and mid-Tertiary stratigraphies between these two areas argue for the presence of a synformal, gently east-dipping normal fault above the Whipple fault in the central Chemehuevi Valley. This fault accommodates 15 km of the total 40-km extension along the Whipple-Chemehuevi detachment fault system. In addition, tilting patterns of upperplate strata and the presence of an areally extensive syn-extension landslide (>70 km2) show that (1) the Whipple detachment fault was active at low angles ( 70 km2) from the north side of the Whipple Mountains; its emplacement occurred before the end of detachment faulting.

INTRODUCTION The distribution, and occurrence, of sulphides and opaque oxides as normal accessory minerals or minor constituents in the common igneous rocks has received little detailed attention, although the subject has an important bearing on both petrogenesis and ore genesis. Variations in the species of sulphide and oxide minerals, in their contained metals, and in their different positions in the crystallization sequence of the rock silicates are present and should have a significant bearing on the time and manner of possible metal departure from a magmatic source. The correlation of these variations with different rock types and rock occurrence has given some interesting and unexpected results. Normal igneous rocks, with a considerable range in composition and texture and from many widely scattered localities, were studied in polished sections, supplemented, in a few cases, by thin-section study. The study was intended to outline and generally survey the field rather than to . . .

INTRODUCTION Red Hill, New Hampshire, received attention from early geologists who worked in this region,1 and it was the subject of an extensive paper by Pirsson and Washington.2 Although certain of the rock types were adequately described in these papers, it is apparent from field examination that the distribution of the rocks has not been shown in sufficient detail and that several of the rock types have never been described. The present paper, therefore, includes a more detailed map than has heretofore been presented, descriptions of the previously undescribed rocks, and some discussion of the relationships of the different types of rocks. ACKNOWLEDGMENTS The six new rock analyses here presented were made possible by a grant from the Penrose Bequest of the Geological Society of America. The writer wishes to express his sincere thanks to the Society for its generosity. He also wishes to thank Professor Esper S. Larsen and . . .

Although numerous previous measurements of the radioactivity of terrestrial materials have been made, most of these researches are of qualitative value only in their applications to geology, geophysics, and cosmology because of inadequate recognition of the analytical care necessary in order to obtain reliable results. In the present study of the radioactivity of terrestrial materials, a systematic program of standardization, calibration, and interchecking has been followed throughout. As part of an international intercalibration among the various workers in this field and a general program of helium age research, several hundred radioactivity measurements have been made. These results represent the most reliable collection of radioactivity determinations which have yet been made within the range of concentrations involved. By combining these newer measurements with the limited number of well-authenticated earlier analyses available, average values have been obtained as follows: 1.37 ± 0.17 × 10 −12 g Ra/g for 43 acidic igneous rocks, 0.51 ± 0.05 × 10 −12 g Ra/g for 7 intermediate igneous rocks, 0.38 ± 0.03 × 10 −12 g Ra/g for 54 basic igneous rocks, and 0.57 ± 0.08 × 10 −12 g Ra/g for 28 sedimentary rocks; 3.0 ± 0.3 × 10 −6 g U/g, 13 ± 2.0 × 10 −6 g Th/g, and a Th/U ratio of 5.0 for 26 acidic igneous rocks; 1.4 ± 0.2 × 10 −6 g U/g, 4.4 ± 1.2 × 10 −6 g Th/g, and a Th/U ratio of 2.6 for 6 intermediate igneous rocks; 0.96 ± 0.11 × 10 −6 g U/g, 3.9 ± 0.6 × 10 −6 g Th/g, and a Th /U ratio of 4.0 for 34 basic igneous rocks. These values are substantially lower than those obtained by Jeffreys in a compilation of most of the measurements reported prior to 1936. The present averages show a more marked decrease of radioactivity with increasing basicity, the Th/U ratios are considerably greater than those compiled by Jeffreys and are in better agreement with those to be expected from geochemical considerations. Two ultrabasic rocks were found to have radio-activities comparable to the low values for iron meteorites. Specific inaccuracies in earlier investigations have been, discovered. Estimates are made of the rate of production of heat by radioactive decay based on the above average values for the different rock types.

The area of an alluvial fan is approximately proportional to the 0.9 power of the area of the drainage basin discharging to the fan. The constant of proportionality is, in part, a function of the erodibility of the bedrock in the source area. By using multiple linear-regression techniques to analyze this functional relationship in three groups of fans in Death Valley, California, and vicinity, we determined coefficients of relative erodibility for some of the dominant rock types. Comparison of these coefficients with weathering characteristics observed in the field and with changes during weathering experiments in the laboratory suggests that fracture density is more important than mineralogy in determining rock erodibility.

INTRODUCTION The origin of green rock complexes in the Adirondacks, their interrelations, and their relations to other Precambrian rock types are obscure. These green rocks have been called syenites (Smyth, 1897), older granites (Buddington, 1939), and charnockites (Berry and Walton, 1962). Buddington (1939), after working in the northern Adirondacks, thought that anorthosite and two generations of granite intruded the Grenville sequence of sediments. De Waard and Walton (1967), after working in the southern Adirondacks, thought that the sequence of sediments was deposited on meta-anorthosite and charnockitic gneiss. Buddington thought that all granitic and charnockitic rocks in the Adirondacks were intruded. De Waard and Walton thought that some belonged to the basement complex upon which the Grenville sediments were deposited, some were initially deposited as Grenville sediments, and some were produced by local melting. The diverse characters and occurrences of granitic and charnockitic gneisses in the Adirondacks show that these rocks formed by several processes. Recent field studies have investigated two green rock complexes: the Tupper Complex (Crosby, 1968; Davis, 1969; Seifert, 1978) and the Diana Complex (Hargraves, 1969; Wiener, 1979). These studies have generally supported Buddington9s conclusions. The Stark Complex is the only green rock complex in the northern Adirondacks that has not been examined since de Waard and Walton published their interpretation of Adirondack geology. Rocks around Jennings Mountain, Lyon Mountain, Taylor Pond, and Loon Lake and between Lake Titus and Blue Mountain are part of the Stark Complex. This paper presents the results of a study on the Stark Complex around Dexter Lake and relates that complex to other green rock complexes, the Grenville sequence, and granitic gneisses that Buddington (1939, p. 135–149) labeled the younger granites.

Gamma-ray spectrometry has been used to measure potassium (K), uranium (U), thorium (Th), heat generation ( A ), Th/U, and ( A – Ak )/K ( Ak = heat from potassium) for over 600 samples in the Idaho batholith. Values of heat generation are remarkably consistent within a given pluton, but vary markedly and predictably between plutons of different composition and(or) different levels of emplacement. Values of ( A – Ak )/K are similar for suites of rocks that were emplaced at approximately the same depth during the same intrusive event, but are considerably different between suites belonging to different intrusive events emplaced at different depths. Thus, on the basis of their ( A – Ak )/K ratio and various geologic parameters, the rocks of the Idaho batholith can be divided into four intrusive groups. Table 1 lists these groups with their characteristics, weighted according to areal abundances of the constituent plutons. The remarkable uniformity of heat generation for a given igneous unit, even over large geographical areas, and the uniformity of ( A – Ak )/K. for rocks intruded during the same intrusive event suggest that these parameters are diagnostic properties of igneous rocks, and may be used by the geologist to map individual units, to correlate different rock types intruded during the same intrusive event, and to separate similar rock types intruded during different intrusive events.

The Precambrian Malvernian complex of the Malvern Hills and the Johnston complex of south Pembrokeshire contain basic to acid igneous rock of similar chemical composition and of calc-alkaline type. Both complexes contain minor rock types of hybrid origin and rock which is enriched in potash. The most important difference between the two complexes is the potash-poor nature of the Johnston and the potash-rich nature of the Malvernian igneous rock. This difference may be explained by the position of the complexes in relation to the contemporaneous orogenic activity suggestive of Precambrian subduction.

Diamictite in the Florida Mountains is about 12 m thick, is probably of younger Precambrian age, and consists of igneous and sedimentary clasts as much as 1.2 m in diameter in a fissile shale matrix. The lower 9 m probably formed from ice-rafted pebbles, cobbles, and boulders that were dropped into marine mud. The clasts were not locally derived, as they consist of rock types not present in the older Precambrian rocks of the Florida Mountains. The upper 3 m appears to be a conglomeratic arkose derived locally after faulting of the lower part of the diamictite.

Potassium-argon and rubidium-strontium age measurements have been made on mica separated from many rock types of the Manhattan Prong and New Jersey-New York Highlands (Reading Prong). Most of the rocks from the Manhattan Prong were dated within experimental error at 360 m.y. (million years). Since this age is identical with those obtained by the U-Pb method by other workers on intrusive pegmatites, it probably represents the last major metamorphic event in the area. The isotopic age of mica in the Highlands at least 10 miles from the zone where complete re-crystallization took place 360 m.y. ago is about 840 m.y. Apparent ages of mica in a transition zone range between these two. The ages in the transition zone are determined by the extent of re-crystallization which in turn is strongly dependent on the local structure. If the 1150-m.y. zircon age obtained by Tilton and others (1960) in the Highlands is the primary age of the basement in this region, major metamorphic events took place at 1150, 840, and 360 m.y. Relict ages as old as 480 m.y. in the Manhattan Prong suggest a fourth event if the rocks of the Prong were deposited in the Cambrian or later. This study illustrates both the difficulties and the effectiveness of isotopic geochronometry in areas of complex metamorphic history.

A metasomatic origin seems established for large parts of the granite phacoliths in the northwest Adirondack Mountains, New York. This conclusion is based upon the discovery and detailed mapping of a blurred but widespread stratigraphic sequence in the phacoliths. Highly complicated patterns of relict beds are defined by alternations of granitic gneiss, amphibolite, oligoclase-quartz gneiss, and a predominant alaskitic granite. Regional reconstructions indicate these relict beds comprise a major basal formation, probably of arkosic and calcareous quartzites, in the exposed Grenville metasedimentary rocks. The existing amphibolite interlayers formed early in the metasomatic epoch, probably replacing the more calcareous quartzite beds. This mafic metasomatism was overlapped and followed by pervasive granitization of the arkosic and highly quartzose members. Pink alaskitic granite is the final metasomacic product. It tends to replace all pre-existing rock types, especially in the cores of the antiforms. This mafic and granitic metasomatism occurred during the evolution of the major folds in the metasedimentary sequence. As the folds evolved, rising domes in the quartzite probed upward into successively higher horizons in the overlying marbles. At least three quartzite antiforms (California, Clark Pond, and South Edwards phacoliths) punctured the overlying marble, invading the basal layers of the Adirondack paragneiss. The crests of the evolving folds in the basal quartzite, capped by carbonate-rich marble, acted as traps for aqueous and carbonated, alkali-bearing fluids, and perhaps some associated anatectic granitic magma generated in the deeper, hotter basement to the Grenville.

Compton made a detailed study of the Bald Rock batholith, California, and suggested that the tonalites and granodiorites of the rim of the batholith originated through contamination. He believed that trondhjemite magma, represented by rocks in the core of the batholith, assimilated meta-basaltic country rocks. This paper reports a detailed study of 22 rocks from the batholith, including all the main rock types. Values for Qu, Or, Ab, and An calculated from the modes show that the rocks fall along a trend slightly below the cotectic boundary between quartz and feldspars and directed toward this surface. Compositions calculated from the modes indicate that formation of the tonalites by contamination is possible but requires improbably high proportions of metabasalt xenoliths in the rim of the pluton. Habits of zircons separated from the rocks show a distribution pattern which emphasizes contrasts between trondhjemitic core and granodiorite-tonalite rim of the batholith but does not support the idea of formation of the mantle rocks by contamination. Dimensional zircon data indicate that zircons of the core are unimodal, whereas those of the mantle of the batholith are bimodal. Comparisons with the Bald Mountain batholith, Oregon, demonstrate many similarities, as well as some striking contrasts. Compositional trends of tonalite to granodiorite are different for the two plutons. Zircons are the same throughout the Bald Mountain batholith. The writers conclude that both plutons were emplaced by forceful injection. At Bald Mountain, Oregon, a uniform magma was injected, but at Bald Rock, California, a migma-magma was emplaced. The trondhjemitie core of the Bald Rock batholith was mainly liquid at emplacement, but the tonalite-granodiorite rim consisted of mobile, viscous solid phases with interstitial melt. The liquid line of descent at Bald Mountain, Oregon, was from tonalite to granodiorite and at Bald Rock, California, only from trondhjemite to leucotrondhjemite.

The major features of the Moon (marcs, highlands, craters, fractures) are briefly reviewed to encourage geologists to participate in the solution of lunar problems—problems involving the interpretation of varied land forms, the rock types and their distribution, potential ore deposits, and lunar tectonics. Special attention is given the fracture systems, the origin of the lava filling many closed craters, and the origin of the central peak complexes.

An isopleth map shows the distribution of a variable by lines of equal magnitude called isopleths. Till-pebble isopleth maps show the areal distribution of pebbles of different rock types in till. Four till-pebble isopleth maps were prepared during a reconnaissance study of the Pleistocene geology of northeastern Montana and northwestern North Dakota. Three of the maps are concerned with erratic pebbles; the fourth, with locally derived pebbles. The isopleth maps of the erratic pebbles reveal an area in northwestern North Dakota where the erratic content of the till differs significantly from that of the till elsewhere in the region. Lithologic relations are described which suggest that the drift of this small area may differ in age from that to the north and south. A till-pebble isopleth map of distinctive, locally derived fluvial pebbles reveals three large areas of till in which these pebbles are abundant. Two of the areas, southwest and west of extensive exposures of the source gravels, suggest that the ice moved generally southwestward across the area. The third area is unrelated to known gravel exposures and may indicate a body of gravel below the till. The study indicates that till-pebble isopleth maps may sometimes prove useful in suggesting the possibility of multiple drifts and in helping to delimit the drifts once substantiated. They may also provide information on direction of ice movement and on the identity of concealed bedrock.

Thrust and strike-slip faulting are recognized in the Plomosa Mountains, southwestern Arizona. The distribution of rock types and the geometry of the thrust faults necessitate that the upper plate moved from east to west. The amount of displacement is not known, but is considered to be large. Apparent separations along the strike-slip faults are in a right-lateral sense and are greater than 19,000 ft. Rhyodacite flows, dated by K-Ar methods at 19 to 20 m.y., unconformably overlie the thrust faults and are cut by the strike-slip faults.

The 38-km-diameter Richat Dome, in central Mauritania, has been widely regarded as a possible astrobleme. Our field examination of the structure and subsequent petrographic study of the critical rock types failed to reveal a single feature attributable to shock processes. We, therefore, dismiss an impact origin for this structure. It must, instead, be endogenic. The same conclusion applies to the much smaller, associated, Semsiyat Dome.

Differentiation of the Elzevir magma into eight phases progressed with the concentration of radioactive elements in later, more acidic members, and culminated with relatively radioactive pegmatites and weakly active veins. The distribution of radioactivity in related rock types is such that it has correlative value. The results of several hundred determinations of radioactivity have shown a tendency for the heaviest elements to concentrate in the periphery of batholiths or stocks. In intrusives greater than 2 miles across, a core zone of lower than average radioactivity may be present if erosion has removed the roof. Several variables must be considered if sampling for radioactivity research is to be representative. Regional differences in radioactivity suggest a steeper geothermal gradient in southeastern Ontario than in western Quebec. Local differences in the radioactivity of batholiths may be important in influencing the course of differentiation, syntexis, and refusion at depth.

Keweenawan basalt analyses having high Al2O3 and CaO contents fall on a linear trend on a CaO versus A12O3 plot. Using plagioclase and augite compositions from the gabbroic anorthosites of the Duluth complex, it is possible to explain the basalt trend by separation of gabbroic anorthosites from a melt rich in A12O3 and CaO. Calculations of the amount of solidification indicate that as much as 60 percent solid material separated before other gabbroic rock types crystallized from the melt. Trends of MgO, FeO, and K2O suggest that another fractionation may have preceded the separation of gabbroic anorthosite. An important conclusion from these arguments is that the multiple intrusions of the Duluth complex represent the crystalline material in magma chambers from which the Keweenawan flows of Lake Superior's North Shore were derived.

Sources of 1,852 basal till pebbles from sites in east-central New Hampshire were identified. Measurements of pebble size, shape, roundness, breakage, lithology, weathering, and distance from source were determined. Roundness, breakage, and size distributions of the pebbles, when compared to their distance from source, indicate that a state of dynamic equilibrium is reached between the processes of abrasion and crushing. The average pebble survives long enough to abraid to a roundness of .5 on Krumbein9s scale, then is crushed again. The early attainment of this equilibrium, within a mile of the source, suggests that the clasts go through at least several cycles of crushing and abrasion per mile of transport. After many miles of transport, certain pebble shapes gradually evolve as stable forms. Spheres are the most stable, discs and rods less so (in that order), and blades are least stable in the subglacial environment. Comparisons to experimental data for the same rock types suggest that abrasion and crushing are about equally effective mechanisms in the production of subglacial till. The pebbles appear to be deposited in multiples of 6 to 7 mi from their source.

Research on amphibolite problems has mainly been concentrated on the chemical differences between ortho- and para-amphibolites with little attempt at a classification of rock types. Therefore, a simple classification of amphibolites ( sensu lato ) is presented to include amphibolites ( sensu stricto ), plagioclase amphibolites, and quartz amphibolites.