8 INTRODUCTION. 



is indicated by the direction in which the coarser beds 

 set in. 



We may note here that the formation is likely to be 

 thickest along the tract where shales and sandstones 

 alternate with each other, and as sandstones are more 

 rapidly accumulated than clays, the formation may thicken 

 landwards by the intercalation of sandstones ; so that if 

 only this portion of a group of beds is preserved to us, the 

 direction of the land will be indicated by the thickening 

 of the sandstones and the thinning of the shales. Among 

 the Palaeozoic rocks this is sometimes the only kind of 

 evidence we possess to guide us toward the position of the 

 land areas. 



The Evidence of Unconformities. The existence of a 

 widespread unconformity in any district, accompanied by 

 the absence of certain groups of rocks which occur in 

 neighbouring districts, raises the presumption that the 

 first district was a land tract during the period which these 

 rocks represent. Unconformities are therefore very im- 

 portant guides in the restoration of ancient geographies, 

 because we may regard the eroded surface as the actual 

 relic of an ancient land. It does not, of course, present 

 all the physical features of that ancient land, because the 

 surface has always been greatly modified as its successive 

 levels came within the erosive powers of the sea under 

 which it sank. Still we can often tell whether it was ori- 

 ginally a high and hilly land, or whether it was of no great 

 elevation. 



In drawing inferences, however, from the absence of 

 certain formations above a surface of unconformity, some 

 caution must be exercised. If an unconformity occurs 

 between two systems which are in geological sequence, 

 such as the Ordovician and the Silurian, or between two 

 parts of the same system, we must conclude that the older 

 set of rocks was elevated during the epoch which is repre- 



