DISTINCT GLACIAL EPOCHS. 75 



below a superficial layer which is not oxidized, would be no 

 proof that there were not distinct ice epochs, since the ice of any 

 later epoch, if such there were, might have planed off the sur- 

 face of the drift left by its predecessor to the depth of the 

 weathering. The preservation of such surfaces after a second 

 ice invasion must be regarded as the exception rather than as the 

 rule. There is always the possibility, too, that an oxidized and 

 weathered zone marking the surface of an older drift sheet 

 exists, where excavations have not opened full sections of drift 

 to view. The absence of weathered zones of drift beneath the 

 surface, or the absence of knowledge of their existence, is 

 therefore at best no more than negative evidence. The absence 

 of greater weathering of the drift outside the limit of the drift 

 supposed to belong to a later epoch, would be positive evidence 

 against the reference of the two sheets of drift concerned to 

 different epochs. 



A specific part of the above line of evidence may be sepa- 

 rately mentioned. One phase of weathering is the disintegration 

 of boulders, and this is a point which can be readily applied 

 even by those who are not geologists. If the boulders of one 

 region are much more commonly disintegrated than those of 

 another, and if the two regions are separated from each other 

 by a well-marked boundary line, the inference lies close at hand 

 that the boulders in the one case have been much longer exposed 

 to disintegrating agencies than in the other. It is no answer to 

 this argument to say that the materials lying at the very front 

 of the drift deposits contain boulders which were derived from 

 the disintegrated rock over which the ice has passed, and that 

 they were therefore in a less firm state at the outset. In many 

 cases these boulders have come from great distances, and com- 

 ing from great distances they must have come in a firm and 

 solid state, else they could not have suffered such extensi\'e 

 transportation, except indeed their position was superglacial 

 throughout their whole journey. This argument has equal force 

 when applied to the area covered by the two sheets of drift 

 where two exist. If within the region of drift under investiga- 



