312 The American Naturalist [April, 



inch. These laminations are sometimes symmetrical, straight and 

 parallel. At other times they are undulatory, and in instances they 

 are greatly curved and contorted in an intricate fashion." 



It was observed that the debris bearing layers were parallel to the 

 base of the glacier and were confined to its lower 50 or 75 feet, with 

 some few exceptions. Even at the border of the glacier clean layers of 

 white ice above the debris strata constituted one-third or more of the 

 section. This is contrary to the view that the debris habitually works 

 up to the surface and forms a layer there as it nears the border of the 

 glacier. 



Prof. Chamberlain was fortunate in being able to observe the process 

 of introduction of debris in progress. At a point in the Gable glacier 

 there was found an embossment of rock over which the ice was forced 

 to pass and in so doing to rise in a dome-like fashion. One side of the 

 dome was melted away, revealing operations at its base. Combining 

 a number of observations, the author gives the following interpretation 

 of the process : 



" The bottom layer of the ice in passing over the crest of the em- 

 bossment would be pressed with exceptional force upon it, and would* 

 as a result, be especially liable to detach fragments from it and imbed 

 them within itself. If debris were being pushed or dragged along 

 between the ice and the rock surface beneath, it would be pressed into 

 the ice and the ice compacted about it with exceptional force. As any 

 given portion of the basal layer passed beyond the crest of the emboss- 

 ment, the vertical pressure would tend to cause it to follow down the 

 lee slope, while the horizontal thrust of the moving ice would tend to 

 force it straight forward. If any given portion yielded to the first and 

 passed down the slope, it would produce a curve in the hardened basal 

 layer of ice. As a result of this, the horizontal thrust, instead of continu- 

 ing to act along the disadvantageous curved line, and against the superior 

 friction of the bottom, would be disposed to cause the layer to buckle 

 at the bend. The fold so formed would be elongated and appressed by 

 the continuation of the process and become a layer. The ice, beneath, 

 however, would gradually yield, and the debris layer would settle down 

 out of the line of maximum thrust and the conditions for a new fold be 



Cases of true faulting and overthrust were seen, the rocky debris 

 being carried along the fault plane. 



As to the method of movement, Prof. Chamberlin presents evidence, 

 which taken in connection with the intrusion and interstratification of 

 earthy material, would seem to indicate that these glaciers move, in some 

 notable part at least, by the sliding of one layer upon another. 



