THE NEOZOIC AGES. 271 



these ways. In like manner, the ancient boulder 

 clay may have been formed in one or more of these 

 ways in any given locality where it occurs, though it 

 may be difficult in many instances to indicate the 

 precise mode. There are, however, certain criteria 

 which may be applied to the determination of its 

 origin, and I may state a few of these, which are the 

 results of my own experience. (1) Where the boulder 

 clay contains marine shells, or rounded stones which if 

 exposed to the air would have been cracked to pieces, 

 decomposed, or oxidized, it must have been formed 

 under water. Where the conditions are the reverse of 

 these, it may have been formed on land. (2) When 

 the striations and transport of materials do not con- 

 form to the levels of the country, and take that direc- 

 tion, usually N.E. and s.w., which the Arctic current 

 would take if the country were submerged, the pro- 

 bability is that it was deposited in the sea. Where, 

 however, the striation and transport take the course of 

 existing valleys, more especially in hilly regions, the 

 contrary may be inferred. (3) Where most of the 

 material, more especially the large stones, has been 

 carried to great distances from its original site, 

 especially over plains or up slopes, it has probably 

 been sea-borne. Where it is mostly local, local ice- 

 action may be inferred. Other criteria may be stated, 

 but these are sufficient for our present purpose. Their 

 application in every special case 1 do not presume to 

 make; but I am convinced that when applied to 

 those regions in Eastern America with which I am 



