AUTOCLASTIC ROCKS * 531 



Falls, where several such layers are visible. It is also well shown 

 in the fine calcikitytes of the Solnhofen beds (Jurassic) of Ba- 

 varia, a formation shown by many characters to be of shallow water 

 origin. (See further. Chajjter XX.) 



Glacial Deposits. 



If we consider ice as water in the solid form, glacial deposits 

 must be classed under the hydroclastic division of rocks. It is 

 manifest, however, that the essential features of glacial deposits are 

 those which most strongly differentiate them from ordinary hydro- 

 elastics, be they fitivial, lacustrine, or marine. In some characters 

 they may resemble atmoclastic and in others autoclastic deposits, but 

 their most marked characteristics are not shared with any other 

 type. Nevertheless, the method of formation of the most typical 

 glacial elastics is closely analogous to that of autoclastic fault brec- 

 cias. Both are produced by the movement of one rock mass over 

 another, for ice in this relation has the essential characters of a solid 

 rock, and both furnish material for the resulting rudyte or finer 

 rock, though that furnished by the ice is readily destroyed. It is 

 true that material not produced by mutual attrition of the two rock 

 masses, the basal rock and the ice rock, may be, and commonly is, 

 incorporated in the subglacial material, but this merely points the 

 essential peculiarities of this autoclastic rock. It is also true that 

 the movement of glaciers cannot be directly compared with that of 

 rock masses involved in faulting, in so far as it is a gliding partly 

 due to internal forces as well as to gravity. Still the essential fact 

 remains, that clastic material is produced by the movement of one 

 rock mass over another and that both rock masses contribute from 

 their substance to the production of this material. In this respect, 

 then, glacial debris may be compared, on the one hand, with that 

 formed by gravity faulting, and. on the other, with that formed 

 by gliding masses resulting in the production of intraformational 

 breccias. Since the ice also moves by constant internal readjust- 

 ments from recrystallization. the products of such motion may to a 

 certain extent be compared with enterolithic deformations. 



Characters of Modern and Pleistocenic Glacial Deposits. The 

 material resulting from the plucking and abrasive action of the ice 

 varies in grain from the finest rock flour to boulders of almost 

 unlimited size. Except where disintegration of the rock surface 

 previous to glaciation has occurred, the fine material will all be 

 rock flour with clay notably absent. The arenaceous material also 

 shows in its angularity and sharpness of grain that it has been 



