112 



number of beds, with nothing but marine remains. It would 

 therefore appear that at the period of the deposition of the 

 lower sandstones and shales, what had previously been sea had 

 become subject to the domain of fresh water, which drifted 

 from some neighbouring land many terrestrial plants — that the 

 sea for a while had again the sway — then fresh water a second 

 time, and at length the sea became sole possessor, and continued 

 so for a long period. These facts the author thought could be 

 best explained by supposing that an elevation of the sea's bed 

 took place after the period of the inferior oolite — then a depres- 

 sion of no great extent, during which the grey limestone was 

 accumulated — then a second elevation ; and finally, after the 

 deposition of the upper sandstones and shales, a permanent 

 subsidence. The point which he had specially investigated 

 was the direction from which the above sandstones had been 

 drifted. 



This can be ascertained in almost every sandstone rock by 

 carefully attending to certain points of structure. If a bed be 

 deposited in tranquil water, the strata are level and horizontal ; 

 but, if a slight current is present, ripples are formed on its sur- 

 face. These, on an average, tend perpendicular to the line in 

 which it moves ; and hence, by observing the direction of the 

 ripples in any rock, the line in which the current ran is known, 

 but not the side from which it came. If, however, its velocity 

 be somewhat greater, the sand is drifted along at the bottom, 

 and produces a kind of rippled structure, in which the layers 

 are thrown down on that side of the ripples towards which the 

 current flows. This constitutes what the author calls ripple 

 drifted, and, by it, the direction from which the current came 

 can be ascertained. When this ripple drift becomes very large, 

 from the increased velocity of the current, it passes into what is 

 commonly called false bedding ; when layer after layer is 

 thrown down in advance of the others, inclined at an angle of 

 about 30' to the true bedding. This is what the author calls 

 drift bedding. When these peculiarities of structure are seen 

 in progress in modern sand drifts, it is to be observed that the 

 slope of the talus is not invariably perpendicular to the line of 



