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Although Symons' (1973) study of the Topley Intrusions is an important contribution to unraveling the Jurassic polar wander path for North America, several difficulties become apparent on reading his paper. He states that a major object of his study was to add data to help clarify the Jurassic portion of the polar wandcr curve. However, he is apparently unaware of our contribution in that area, which was published last year (Steiner and Helsley 1972). Quite to the contrary of his assertion that no confidence limits are given for the Kayenta and Sumrnerville poles, in that paper all relevant statistical information is given for the data of our earlier abstract (Steiner and Helsley 197 1 ) . In comparing his results to other published Jurassic data, Symons makes several inaccurate statements regarding the White Mountain Magma Series data (Opdyke and Wensink 1966), referring to it as reliable and saying that it predates the Topley pole by 40 m y . Since many workers, in comparing their data to published data, consider the White Mountain result reliable, issue is taken here with that concept. As was discussed in our paper, the White Mountain Series pole: ( I ) is not 180 m.y. old, ( 2 ) spans a time period of up to 100 may., from 199 to 96 m.y. (Foland 1969; Foland et al. 1971; Armstrong and Stump 1971 1, and (3) is derived from widely scattered site mean poles, indicative of either secondary component remaining or individual plutons having been magnetized at different times during the long time between Triassic and Cretaceous spanned by the intrusions. The White Mountain data are suspect in that the normal results in the mixed polarity study are associated mainly with monzonites and all give a present pole position while the reversed results are all from gabbroic compositions and give a streak of poles away from the Cretaceous pole. Therefore, the White Mountain pole cannot be taken to represent a reliable Jurassic pole to which other data from North America can be compared. Furthermore, the White Mountain pole does not predate the Topley Intrusions but includes them within its age span. Since the currently accepted age for the boundary of the Jurassic and Cretaceous Periods is 136 m.y. (Geol. Soc. Phanerozoic time-scale 1964), the Topley Intrusions, within their error limits (139 * 4 m.y.), fall on the Jurassic-Cretaceous boundary. Therefore, Symons' statement that his pole position, intermediate between those of the Jurassic and Cretaceous, is "contrary to the static TriassicJurassic pole position interpretation of Steiner and Helsey (1971 ) " makes no sense. The age of his pole position is at the end of the period to which we referred. Moreover, we would likc to reiterate our findings (Steiner and Helsley 1971; 1972) that the pole relative to North America apparently remained in its Triassic position until post 150 m.y. (Summerville time) and then moved very rapidly to its Cretaceous position. New data of ours to be published from uppermost Jurassic sedimentary rocks of the Colorado Plateau support that conclusion. Although the Topley Intrusions pole has a rather large confidence ovd, the position of the pole is in agreement with the concept of rapid movement in the time post 150 m.y. (His pole, at about 139 m.y., is almost half way between Triassic and Cre

Mothersill and Fung (1972) declare in the introduction of their paper that one of the aims "of this investigation was to determine the Quaternary stratigraphy . . . of the northern part of the Lake Superior basin". Therefore, a reader expects not only new original data, but also consideration of generally accepted stratigraphic criteria including the code of the American Commission on Stratigraphic Nomenclature ( 1961 ), and correlation with up-todate stratigraphic schemes of other authors. Unfortunately, the authors are confusing rock stratigraphic with time stratigraphic terms, are defining a time transgressive contact as the boundary between two established time stratigraphic units, and are not using the latest references for correlations. To some extent, this has been done already before by the senior author (Mothersill 197 1 ) , and the present paper of Mothersill and Fung (1972) repeatedly refers to his equally disputable stratigraphic statements. In their Table 1, Mothersill and Fung (1972) call Farrand's (1969) names of sediments encountered in cores of Lake Superior as "Time Stratigraphic Units". They could be considered informal rock stratigraphic units, but never time stratigraphic. No reasons are given why, in the same table, the authors (and also Mothersill 1971) have chosen Hough's (1958) very seldom used classification of the Wisconsin Stage, when they already indicate in the table, that most of Hough's substages (five out of six) are missing in the area; the only one listed as present (Valders) is not a term introduced by J. L. Hough. Why not refer to any other more recently published and widely used classifications of the last glacial stage in the Great Lakes Region? No explanation is given by Mothersill and Fung ( 1972) and Mothersill (1971 ), why they prefer 11 000 yr B.P., based upon a few black carbon dates (Flint and Deevey 1951) for dating the Valders advance, rather than using, or at least mentioning the 18850 + 100 yr B.P. average date (Broecker and Farrand 1963) of the Two Creeks forest bed, which was flooded and the trees killed, during the Valders advance. The latter date is based upon a special investigation of new material and comparison with more than twenty other radiocarbon dates. One of the most confused concepts in Mothersill and Fung (1972) is the HolocenePleistocene boundary. They say that Mothersill ( 1971 ) , "defined the basal contact of the claysilt sequence with the underlying gray varves as the Holocene-Pleistocene boundary". They also state that this contact represents an unconformity. Comparison of their Tables 1 and 3 shows that chronologically this unconformity is partly in the Pleistocene, partly in the Holocene: in Table 3 the extrapolated dates of basal Holocene range from 5400 + to 13 000 2, while in the 'Time Scale' of Table 1 the date of 11 000 yr B.P. is included in Pleistocene. The Holocene-Pleistocene boundary is a timestratigraphic term and therefore it can not fluctuate over a wide span of time. It may be noted that the INQUA Commission on the Holocene announced at the VIII INQUA (International Union for Quaternary Research) Congress in Paris, 1969, that "For the reference boundary between the Pleistocene and Holocene with world-wide validity, the Commission proposes a date at about 10,000 B.P. measured in radiocarbon years" (Hageman 197 1 ) . Neither Mothersill ( 1971 ) nor Mothersill and Fung (1972) have proven that the contact between the clay-silt and underlying varves in

During June and July of 1968 the Geological Survey of Canada participated in the llominion Observatories crustal seismic experiment centered on the (3renvllle front in north central Quebec. Participation was to the extent of recording the shots of the experiment at locations subadjacent to the Appalachian front from Kimouski and IiiviPre Du I,oup, Quebec to Edmundston, New Brunswick. Recordings from a three component station were taken on photographic paper, and also on frequency modulated magnetic tape. A preliminary study of 19 Pn first arrivals results in velocities for the upper mantle ranging from 8.08 km/s to 8.23 km/s with a mean of 8.15 km/s under the shots. Under the stations Pn events show velocities for the upper mantle ranging from 7.81 kn~/s to 8.65 km/s with a mean of 8.15 km/s. A least-squares time-term analysis for Pn events results in a velocity oi 7.94 -t 0.42 km/s at the 90% confidence level. This solution shows large residual times ranging between -+0.7 s which probably imply complex geological structures near the shots and the recording stations. Time and distance data are summarized as well as details of computations. Pendant les mois de juin et juillet 1968, la Commission Gtologique du Canada a pris part aux expiriences sismiques efiectuecs sur la crofite terrestre par lcs Observatoires du Dominion et centrCes sur le front Grenville du nord du QuCbec central. Sa participation s'est limitCe h I'enregistrement des tirs de llexpCrience h des localites adjacentes au front Appalachian et sises de Rimouski et Riviere-du-Loup, P.Q. B Kdmundston N.B. L'enregistrement sur papier photographique et aussi sur bande magnCtique 2 frkquence modulde, fut rCalisC h l'aide d'une station B trois parties composantes. Une Ctude priliminaire des temps de premikre arr~vie de 19 ondes Pn a donnC des vitesses de propagation allant de 8.08 km/s B 8.23 km/s (la moyenne ktant de 8.15 km/s) dans le manteau supC.rieur, lors des Cbranlements. Sous les stations, on obtient pour le manteau qupdrieur, des vitesses de propagation de ces ondcs Pn allant de 7.81 kmis h 8.65 km/s avec une moyenne de 8.15 km/s. l,'analysc, par la mtthode des moindres carrCs, du terme temps pour les ondes Pn a produit une vitesse de 7.94 2- 0.42 km/s B un niveau de confiance de 90%. Cette solution fait boir des temps rbiduels considtrables variant entre -i- 0.7 s, ce qui indiquerait sans doute des str~ictures gCologiqucs complexes pr2s des points de tir et des stations d'observation. On rksume les donnces sur le temps et les distances ainsi que les artifices de calcul. [Traduit par le journal]

The difficulties of the aerial survey of the Aletsch Glacier lay in establishing ground control in the high mountains and locally on the moving glacier surface, and in plotting featureless névés on the autograph. The major problem in plotting proved to be the delineation of the margins of active glaciers where marginal zones were thickly covered with debris or fresh snow, or where it was necessary to define the boundary between the active glacier and stagnant ice. The distinction between active and stagnant ice is based on movement and nourishment, that is, on characteristics not directly visible in the air photographs, and the boundary is too often a matter of personal judgment. The authors hold that this boundary would be better omitted on an accurate map of the type discussed, while the glacierized area should be represented only in so far as ice is clearly indicated by surface features. A further provisional boundary might be given to indicate suspected buried ice. The paper emphasizes the importance of favorable glacier conditions when the control photographs are taken, to facilitate the mapping and interpretation of glaciological features.

Five glacier maps at scales of 1:5 000 and 1:10 000 were produced by the Photogrammetric Research Section of the National Research Council and the Axel Heiberg Island Expedition of McGill University for use in glaciological research in the Canadian Arctic. After general comments on the need for and scope of glacier mapping, the authors discuss in some detail the main problems encountered. It is maintained that glacier maps belong to the class of 'special maps' and that the specifications for them should be set by the glaciologists.Accuracy requirements, choice of mapping scale and contour interval, delineation of various glacier units, extent of detail plotting, etc. are critically reviewed, The 1:10 000 map of the entire White Glacier catchment area, accompanying this paper, is an example of a map which serves various glaciological purposes, primarily calculations of mass change. To facilitate future glacier mapping, the authors suggest that clear definitions of glaciological terms such as 'active glacier' or 'glacierized area' should be established and that their cartographical presentation should he further standardized.

The Karakoram and Alaska–Yukon region between them account for perhaps 90% of the known surging glacier events. While modern research and information is available for the latter region, the Karakoram, though often referred to, remains virtually unknown except in a very general sense. The prime aim of this paper is to collate all the information that is available for the Karakoram from the usually old and inaccessible documents where it is found. It establishes eleven instances of surges with reasonable certainty and the probability of many more. The (rare) instances of detailed descriptions of the phenomena are quoted. A short discussion is given of the degree to which the information, and knowledge of the Karakoram environment, equate with various mechanisms put forward to explain surging glacier events.

The St. Lawrence valley system (including the St. Lawrence, Ottawa, and Champlain valleys, and the St. Lawrence or Cabot trough) is coextensive with a well-defined pattern of seismic activity. The valley system is in a region of general updoming, normal faulting, and alkaline igneous activity of a distinctive type. The main phase of tectonic activity probably dates back to Mesozoic time. The above and other evidence presented in this paper indicate the existence of a major rift valley system that may be called the St. Lawrence rift system.The Rough Creek – Kentucky River fault zone, and the normal fault zones in Texas and Oklahoma, and the Lake Superior fault zone probably represent extensions of the St. Lawrence rift system. However, current seismicity indicates that the present tectonic activity is along a straight zone running through lakes Ontario and Erie into the Mississippi embayment. The St. Lawrence rift system may also be connected with the mid-Atlantic rift, in the region of the Azores plateau.The rift hypothesis presented may be useful as a regional guide in the search for niobium-bearing alkaline complexes and diamond-bearing kimberlites.Crustal tension in the St. Lawrence region may be genetically related to the opening of the Atlantic basin as postulated in the hypothesis of continental drift.

The paper emphasizes that the choice of a proper procedure is essential for efficient glacier surveying. Terrestrial versus aerial photogrammetry and the use of electronic surveying procedures versus triangulation have been tested in surveys on the Athabasca and Saskatchewan glaciers, and on glaciers in northern Ellesmere Island, southeastern Alaska, and the Yukon Territory. The Otto Fiord glacier survey in northern Ellesmere Island demonstrates that no ground control is needed for volumetric glacier studies. Accurate maps needed for other studies depend on the establishment of control. Control can most efficiently be determined by tellurometer and phototheodolite with helicopter support, as shown in Alaskan glacier surveys. For large glaciers phototopography provides a good means of extending control from tellurometer traverse stations, as demonstrated in the Mount Kennedy survey. Simple one-color maps are an economical means of preserving survey results for future glaciological evaluation.

The recent paper by Roberts and Mark (1970) illustrates most effectively the hazard of depending on a computer and a single field criterion to solve problems in glacial geology. The problem lies not in the computer technique for generalizing trends, but rather in the assumption that fabric analysis of till at a limited number of sites necessarily portrays faithfully a single, simple, and contemporaneous pattern of ice movement; this assumption is risky indeed in mountainous terrain. The hazard is, moreover, accentuated when field data provide only the local trend of ice movement, as the sense of the movement is also necessary to the trend surface analysis. The hazard is clearly illustrated in the example cited by Roberts and Mark (1970) from the eastern Fraser Valley of British Columbia (their Fig. 4 reproduced here as our Fig. 1). In this example till fabric studies have been restricted to deposits mapped by Armstrong (1960) as of Sumas age, a product of a short lived (perhaps 1000 years) glacial stade. Their outermost sites (e.g. Nos 9 and 14) may differ slightly in time of deposition from those nearer the source (e.g. Nos. 8 and 12) and some changes in the ice flow pattern during the period of deposition seem called for (Figs. 4 and 5). Even more important to the analysis is the basis for assuming sense of motion at their sites of fabric studies. Roberts and Mark evidently assumed (p. 1183) that the record was one left by "a valley glacier which flowed (southwards) from the side valley occupied by Stave Lake (the Hatzic-Stave Valley of Fig. 1) into the Fraser lowlands." The present authors, on the contrary, have evidence (see below) that the movement of the Sumas ice was northerly, rather than southerly, in the north

ARMSTRONG, J. E. 1960. Surficial geology of the Sumas MATHEWS, W. H. 1967. Profiles of late Pleistocene Map-area British Columbia. Geol. Surv. Can. glaciers in New Zealand. New Zealand J. Geol. Paper 59-9. Geop. 10, pp. 146-163. FYLES, J. G. 1963. Surficial geology of Horne Lake ROBERTS, M. C. and MARK, D. M. 1970. The use of and Parksville Map-areas, Vancouver Island, trend surfaces in till fabric analyses. Can. J. Earth British Columbia. Geol. Sum. Can. Mem. 318. Sci. 7, pp. 1179-1 184.