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Using a Carnegie Model "C" ionization chamber together with Geiger-Mueller tube coincidence circuits, the nature of the radiation which produces large bursts (greater than 200 particles) of cosmic rays under thick absorbers at 11,500-feet elevation has been investigated at Climax, Colorado. The percentage of this radiation which was accompanied by air showers was determined and found to be dependent upon the burst size. The absorption mean free paths for the burst-producing radiation (excluding bursts produced by $\ensuremath{\mu}$-mesons and by air showers) were measured in lead and iron and found to be 350\ifmmode\pm\else\textpm\fi{}40 g/${\mathrm{cm}}^{2}$ in lead and 240\ifmmode\pm\else\textpm\fi{}20 g/${\mathrm{cm}}^{2}$ in iron. The barometric effect on this radiation yielded an absorption mean free path of 65\ifmmode\pm\else\textpm\fi{}14 g/${\mathrm{cm}}^{2}$ in the atmosphere, and the zenith angle distribution was found to be that due to an absorption mean free path of 75\ifmmode\pm\else\textpm\fi{}14 g/${\mathrm{cm}}^{2}$. These results are shown to indicate that the radiation responsible for these large bursts consists largely of protons and $\ensuremath{\pi}$-mesons of energy greater than 60 Bev.