INTRUSION OF EARTHY MATERIAL. 207 



to disclose the intimate nature of their mass. Definite planes of parting 

 were developed between some of the layers. This was often true even 

 when the la3'ers were not separated by any earthy filament, the ice on 

 both sides being white and pure. The plane of parting between the 

 layers was often slightly gaping at the surface ; sometimes the two layers 

 seemed to be peeling apart. I found it easy by a moderate stroke of the 

 spike of my alpenstock to split blocks along these partings. The sepa- 

 rated blocks presented smooth surfaces, which seemed to leave no ques- 

 tion of their analogy to slickensides. The layers on either side were 

 made up of coarse granules of ice, intimately interlocked, so tliat an 

 attempt to cleave the mass at other points resulted in a fracture of the 

 most ragged and irregular sort. 



But the best evidence of the verity of shearing between tlie ic(;-i)lates 

 lies in the intrusion of the earthy material itself. I was fortunate enough, 

 unless I misinterpret, to observe the actual process of intrusion. The 

 best illustration was found on the north side of a short lobe of the great 

 ice-cap designated the Gable glacier. Just back of the point of observa- 

 tion there was a large embossment of roctk, which expressed itself at the 

 surfiice of the ice b}^ a beautiful halfdome, like the Ilalfdome of the Yo- 

 semite. The other half of the dome was cut away, revealing the opera- 

 tions at the base within. Here it was observed that trains of debris 

 apparently rubbed from the surface of the embossment, were being car- 

 ried out almost horizontally into the ice in its lee. Some of these were 

 short, while others extended several rods into the ice. They were some- 

 what inclined downward, but the slope of the glacier being greater, they 

 passed out into the body instead of following the base of the ice. At 

 one point the overthrust reached such a degree as to carry the eartliy 

 layers obliquely almost across the thickness of the glacier, producing a 

 pronounced unconformity. The illustrations will show these phenom- 

 ena with an accuracy and vividness quite beyond the power of a verbal 

 description. (Figures 9 and 10, plate 7.) 



On the East Branch glacier a similar phenomenon was ol)served below 

 a cataract of the ordinary type. Here tongues of debris, having their 

 origin in the bowlder-clay below the glacier, were seen to reach out into 

 the basal portion of the ice as though tliey were being introduced into it 

 by the differential movement of the layers upon each other. The mode 

 of operation seems to be this : When the ice is forced over a prominence 

 it settles down a little in its lee, and is then protected somewhat from 

 the thrust of the ice behind ; the next ice that passes over, l)eing prevented 

 by the former portion from settling down at once, is thrust forward over 

 it. To some extent this is accomplished Ijy the bending and doubling of 

 the layers and to some extent by distinct shearing. At length, however, 



XXIX-BuM.. GEOt. Soc. Am., Voi,. 6, 1804. f 



