E. Hitchcock on a Bowlder in Amherst, Massachusetts. 399 



and grooving should be deeper, and the edges less rounded, (as 

 they are) than by what I suppose to have been the subsequent 

 processes. 



There is another way in which this striation of the bottom 

 might have been accomplished. It might have been done while 

 yet the bowlder was a part of the ledge from which it was bro- 

 ken. In that case it must have been turned over after starting 

 from its bed. 



A third method may be suggested for this work. After the 

 bowlder got mixed up with other fragments, and a strong vis a 

 tergo, either aqueous or glacial, was pushing them all forward, 

 so large a block as this might have pressed so heavily upon the 

 surface as to be deeply furrowed. 



That a strong force was ex- 

 erted upon the bowlder to 

 urge it forward, is obvious 

 from a fact respecting the end 

 of it, (A) lying towards the 

 north (now the south end), as 

 shown by the annexed out- 

 line. Both ends appear for the 

 most part as if acted upon 

 chiefly by water, being irregu- 

 larly rounded and smoothed, 

 but not furrowed, except in 

 two places, a and b. Near the 

 middle, the top, as may be 

 seen, projects a foot or so, and 

 on each side the surface is 

 striated by lines running up- 

 wards, as if smaller bowlders 

 had struck against it, and not 

 being able to move it, were forced over it. 



If a strong current were thus crowding detritus against and 

 over the bowlder, its oblong form would keep its longer axis in 

 the same direction as the stream. Hence the smaller fragments 

 forced against and over it, would smooth the top and the sides 

 in the same direction. They would press most heavily upon the 

 top, and accordingly the striae are much deeper there than upon 

 the sides, though it should also be recollected that the edge of 

 a stratum is usually harder than its face. 



I impute the parallel striation of this bowlder, then, first to 

 its great weight, which caused smaller fragments to slide over it 

 more or less ; and secondly, to its oblong form, which kept it 

 nearly in the same position while advancing. 



The only striae on this bowlder not yet described, are a few 

 faint ones running- obliquely across the present north end, (the 



