INTRODUCTION. . 7 



reached most of the sediment would subside, and little 

 would be left to travel farther. A deposit formed under 

 these conditions would form a large lenticular mass which 

 would thin out principally in two directions, viz., in the 

 direction of open sea and in the direction of land. Subsi- 

 dence would make no difference to this arrangement, but 

 would only tend to increase the thickness of the deposit. 



The deposit would also thin out laterally, away from the 

 central axis of deposition, but a transverse section across 

 the mass would exhibit a different appearance from a 

 longitudinal section ; in a longitudinal section the size and 

 nature of the particles composing the deposit would be 

 nearly the same throughout, while in a transverse section 

 we should find a gradation from coarse sand to fine 

 mud. 



When, therefore, we are dealing with a deposit, or a 

 group of beds, which is thinning out in a certain direction, 

 no conclusion should be drawn without having regard to 

 their lithological characters, and whether these change in 

 that direction or not. Thus, if we start with a thick mass 

 of shale or clay, the materials of which must have been 

 derived from the land, and we find that this simply be- 

 comes thinner without change of character or replacement 

 by other deposits, we can only infer that we are passing 

 away from the source of supply, possibly toward what was 

 deep water, but more probably in a lateral direction from 

 the major axis of deposition. If, however, as it diminishes 

 in thickness, it becomes decidedly more calcareous, assum- 

 ing the character of a marl and including beds of limestone, 

 we may assume that we are proceeding away from the con- 

 temporaneous land and toward what was then an area of 

 clear and deep water. If, on the other hand, a thick argil- 

 laceous deposit is gradually replaced by beds of sandstone, 

 and these by pebbly and conglomeratic beds, there can be 

 no doubt that the position of the contemporaneous land 



