353 The Glacial Theory of Prof. Agassiz. 



rocks in the channels of rivers are often worn smooth ; but Dr. Buckland 

 contends, and apparently on good grounds, that straight parallel stria 

 and grooves never are, and cannot possibly be, produced by the action of 

 gravel and stones in a stream. The abrading material, say a fragment 

 of rock, if it rolls along, will perhaps make occasional indentations, or 

 now and then an irregular scratch ; but it cannot produce straight, par- 

 allel, continued strice, or grooves, unless held fast by some substance 

 which prevents it from roUing, and gives its motion a determinate direc- 

 tion, as the cutter in a grooving plane is kept in a fixed position by the 

 wood. Now, the ice of a glacier (or iceberg) is an agent which an- 

 swers this purpose admirably ; we see that it actually produces the 

 effects described ; we know no other agent capable of producing them ; 

 and it is therefore inferred that where well defined stricE or grooves are 

 found on rocks, we have evidence of the former existence of moving 

 masses of ice. 



Moraines. — These afix)rd other evidence of the ancient existence of 

 glaciers after they have disappeared. Long terraces or banks of gravel 

 are occasionally found on rocks forming the sides of valleys, high above 

 the bottom, and where the surface they rest on is much inclined. Ge- 

 ologists have felt the difficulty of accounting for these deposits. Their 

 situation is inconsistent with the idea that they were formed by running 

 water ; neither could they be deposited on the margin of lakes, because 

 their parts are often found not to be on the same level. Again, they 

 are found stretching like bars across the mouths of valleys, in situations 

 from which a great current, so far from depositing them, would have 

 swept them away, if they had previously existed. Now, both kinds are 

 well accounted for on the supposition that they were the moraines of 

 glaciers ; those on the sides of the valley being lateral, and those bar- 

 ring up its mouth being terminal moraines. 



Again, we sometimes find one or two long ridges of gravel stretching 

 through a wide valley lengthwise or obliquely, without discovering any 

 thing in the shape of the valley to indicate why the current, if water 

 was the agent, should have accumulated the movable matter here, rather 

 than spread it over the surface. This also is explained, if we assume 

 that it was a raedial moraine. When one valley opens into another, the 

 two lateral moraines on the inner sides unite, and the compound glacier, 

 besides having a line of blocks and gravel on each side, has a third 

 stretching along the middle, and which is therefore called a medial 

 moi'aine. There are examples in Switzerland of glaciers with three, 

 four, or six medial moraines. Now, were the glacier to melt away, 

 owing to a change of climate, these three, four, or six medial moraines 

 would form as many ridges of gravel running along the bottom of tlie 

 valley, or obliquely through it, and would resemble deposits occasion- 

 ally seen in this country. 



