THE CAUSE OF THE GLACIAL PERIOD. 483 



having the waves wash the glaciated surfaces away gradually 

 from the edges inward, we should find merely an encroach- 

 ment made here and there upon the border during a portion 

 of the subsidence, until, finally, when the waters covered the 

 whole, all but a very thin stratum of the upper portion would 

 be protected from further disturbance. Especially must the 

 till remain in the innumerable buried channels of the glaci- 

 ated region, and over the extensive protected northern slopes. 

 It is tints difficult to conceive how there should ever be any 

 such complete removal of the ground-moraine from the im- 

 mense glaciated area of North America as Mr. Croll sup- 

 poses to have occurred several times over in preceding gla- 

 cial epochs. 



The facts supposed to prove, by direct evidence, the ex- 

 istence of glacial periods in the various successive geological 

 epochs, can be briefly stated.* 



Beginning with some of the oldest sedimentary strata, 

 Professor Archibald Geikie has discovered what he believes 

 to be unmistakable signs of glacial action in the north of 

 Scotland, in Sutherland shire, on rocks of Cambrian age — 

 that is, just below the base of the Silurian system. Here he 

 reports extensive surfaces of gneiss rock worn into the char- 

 acteristic "rounded bossy surface" of glaciated regions, and 

 this evidently runs under an extensive deposit of breccia of 

 glacial origin, made up of fragments eroded by ice at that 

 early period of glaciation.f Some of the fragments of this 

 overlying breccia are said to be from five to six feet long. 



A second instance of early glaciation, mentioned by Pro- 

 fessor Ramsay, occurs in the south of Scotland, in Ayrshire 

 and \Vigton shire, in the Lower Silurian formations of that 

 region. Here are extensive sedimentary rocks, containing 



* On the whole, the best summary of the evidence upon this subject, and the 

 one to which we are mainly indebted for the facts here presented, was given by 

 Sir A. C. Ramsay, Director-General of the Geological Survey of the United King- 

 dom, in his Presidential Address before the British Association of Swansea in 

 1880. (See "Nature," vol. xxii, p. 388 el seq.) 



+ See communication to " Nature,'' vol. xxii, pp. 400-403. 



