10 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



and XX, YY, ZZ, etc. The shallower and natter the bottom, 

 the more nearly are the two sets of planes coincident, but 

 they can never coincide whilst dry land and sea both exist 

 in proximity. If, therefore, we are able to discover in the 

 outcrops of strata the degree of discordance, we obtain evidence 

 of the character of the earth-movement at the time of 

 deposition. 



If the subsidence is rapid, the landward shifting of a\ a" 

 and 6', b" in relation to each other and to a, b is respectively 

 greater and they will become much attenuated, partly as a 

 result of the greater area over which a given quantity of debris 

 will be spread, and partly owing to the decreased erosion due 

 to the lowering of the land. The more rapid the subsidence 

 the more obvious will be the phenomenon of overlap, and the 

 greater the amount of transgression. The existence of overlap 

 when the downward movement is gentle and continuous is not 

 necessarily shown by lithology, but may be revealed by the 

 organic remains if the conditions were suitable for development 

 and preservation of life. 



Rapid elevation will clearly give rise to pen contempora- 

 neous erosion of the series of deposits formed when the land 

 stood at a lower level, and the new series of deposits may 

 contain rolled fragments derived from the older series. The 

 plane of junction between the series a, 6, c, d, and a', &', 

 c', d' may appear only as an ordinary bedding-plane, 

 although it is actually of the nature of a disconformity. As in 

 the case of rapid subsidence, the result upon the outcrop will 

 be discontinuity of particular lithological phases. But in all 

 probability the most noteworthy result of the elevation will be 

 the sudden increase in the quantity of detritus due to the 

 activity of the agencies of denudation upon the elevated land. 

 The rapidity of the movement will prevent the coarser materials 

 from being either well-graded or well-rounded. Evidence of 

 penecontemporaneous erosion may well be obscured in the 



