THE NEOZOIC AGES. 269 



and which holds large angular and rounded stones 

 or boulders confusedly intermixed ; these stones may 

 be either from the rocks found in the immediate 

 vicinity of their present position, or at great distances. 

 This mass is usually destitute of any lamination or 

 subordinate stratification, whence it is often called 

 Unstratified Drift, and is of very variable thickness, 

 often occurring in very thick beds in valleys, and 

 being comparatively thin or absent on intervening 

 hills. Further, if we examine the stones contained 

 in the boulder clay, we shall find that they are often 

 scratched or striated and grooved ; and when we 

 remove the clay from the rock surfaces on which 

 it rests, we find these in like manner striated, 

 grooved and polished. These phenomena, viz., of 

 polished and striated rocks and stones, are similar to 

 those produced by those great sliding masses of ice, 

 the glaciers of Alpine regions, which in a small way 

 and in narrow and elevated valleys, act on the rocks 

 and stones in this manner, though they cannot form 

 deposits precisely analogous to the boulder clay, 

 owing to the wasting away of much of the finer 

 material by the torrents, and the heaping of the 

 coarser detritus in ridges and piles. Further, we 

 have in Greenland a continental mass, with all its 

 valleys thus filled with slowly-moving ice, and from 

 this there drift off immense ice-islands, which con- 

 tinue at least the mud-and-stone-depositing process, 

 and possibly also the grinding process, over the sea 

 bottom. So far all geologists are agreed; but here 



