Rev. W. D. Conybeare on the Phenomena of Geology. 259 



have been formed by excavation. Moreover this account of 

 their origin seems to follow as an almost necessary corollary 

 from the phaenomena, of the gravel derived from the water- 

 worn fragments torn from the mass of those strata, which 

 were detailed in the foregoing article: for the existence of 

 this gravel obviously presupposes the partial destruction of the 

 strata which yielded it. 



I shall now, then, more particularly examine the physical 

 structure of the districts which are thus especially affected by 

 valleys of excavation. 



These districts are occupied by strata disposed in planes 

 approximating to the horizontal, from which they seldom de- 

 viate more than 5 or 6 ; their emergence from the super- 

 strata, therefore, usually forms acclivities of gentle slope along 

 the back of the strata ; their termination against the substrata 

 (or, as it is technically termed, their basset edge) presents, on 

 the contrary, an abrupt escarpment, traversing the strata. 

 Beneath these escarpments, therefore, we generally find ex- 

 tended valleys ranging in a direction parallel to the strata, 

 which have usually been distinguished as longitudinal valleys. 

 But besides these, other systems of valleys occur cutting across 

 the ridges presented by the escarpments of the basset at nearly 

 right angles. Now as these strata usually, at one extremity at 

 least of their course, abut against an oceanic basin, the longi- 

 tudinal valleys naturally appear to open one line of drainage: 

 that presented by the transverse valleys is, however, the chan- 

 nel usually pursued by the actual rivers. In my paper before 

 alluded to on the Valley of the Thames, I have shown that its 

 waters thus cut transversely through three chains, which seem 

 to oppose themselves as barriers to their course, although the 

 longitudinal valleys ranging at the base of those barriers ap- 

 pear to open more obvious outlets to the drainage ; and it is 

 quite obvious, that since those longitudinal valleys have ex- 

 isted, the waters could never have risen within several hun- 

 dred feet of the summits of the chains, over which on the flu- 

 vial hypothesis they are once supposed to have flowed. It 

 must be argued, then, that at first no such longitudinal valleys 

 existed ; that is, that the strata did not, as at present, terminate 



Mr. Lyell has urged to some of the arguments which the geologists of my 

 school employ, such as the transport of gravel, &c. in directions contrary 

 to the actual drainage of the valleys, he seems to have overlooked the di- 

 stinction between such districts, and those of inclined and dislocated strata ; 

 for the examples have been taken from the more horizontal districts : to 

 which his remark, that we may suppose the actual drainage to have been 

 altered from that which originally prevailed, by earthquakes, &c. will not 

 apply. 



2 L 2 in 



