290 GEOLOGY. 



age from amid the waters, no such rivers as the Missis- 

 sippi or the St. Lawrence were upon it. And the grand 

 river systems of the earth were not fully formed till after 

 the continents were expanded to their full limits, and 

 the mountain ranges spoken of in 385 were raised up 

 in the Tertiary age. The Post-tertiary age then was, as 

 stated by Professor Dana, " the era of the first grand 

 display of completed river systems of the first Amazon, 

 Mississippi, Ganges, Indus, Nile," etc. 



402. Terminal Moraines of the Ancient Glaciers. The 

 glaciers of the Post-tertiary age had terminal moraines, 

 like those of the present time ( 190). They have, of 

 course, been much altered by the fluviatile operations, 

 that is, the operations of moving water, which produced 

 such great effects during the long ages of the Champlain 

 and Terrace periods, and hence it is that I notice them 

 here. The alterations alluded to are so great that the 

 resemblance of the remains of these ancient moraines to 

 those found at the foot of the glaciers of the present day 

 is not obvious-to the common observer. It is only the 

 practiced eye of the skillful geologist that can discover 

 it. Agassiz, Guyot, and others have investigated this 

 subject by extended observations in some of the locali- 

 ties where the vast glaciers of the Glacial period lay, es- 

 pecially in Switzerland and the island of Great Britain, 

 and have traced out the moraines with signal success, in 

 spite of the obliteration of their distinctive marks by 

 causes which have been acting upon them for ages. 

 They are, for the most part, semicircular in shape, the 

 concavity being toward the direction from which the gla- 

 cier moved downward. Sometimes there are several 

 of these semicircular walls, one placed within another. 

 Each of these is a moraine, the outer one having been 

 formed first, and the others one after another, as the gla- 

 cier retreated or became shortened ( 189). In the pass- 

 ing away of the Glacial age there was, of course, a con- 

 stant diminution of the extent of the glaciers, and conse- 



