from Penigent to Kirkbi/ Stephen. 101 



fossil remains subordinate to them have lived and died on the spots where 

 they are now found. On the contrary, some of the other alternating- strata 

 have originated in an action decidedly mechanical, and their imbedded ligneous 

 fossils have been drifted from a distance. Hence it is that these strata are in 

 general much less regularly continuous than the groups of limestone. The 

 observation must not, however, be strained too far ; as some of the thin bands 

 of coal above described are, on any hypothesis, of astonishing regularity and 

 continuity*. 



5. The bottom beds of each calcareous group are often impure, and contain 

 very few traces of organic remains, and do not generally alternate to any con- 

 siderable extent with shale and sandstone. We may also remark, that the 

 corallines, zoophytes, &c. found in such positions are commonly of small size. 

 At the top of each limestone group, on the contrary, the alternations of shale 

 and sandstone are universal, and the fossils are numerous and full-grown. 

 From all which it seems to follow — that each limestone group commenced at 

 the beginning of a period of repose — that the marine animals which assisted 

 its growth were at first few in number and ill-developed — that they gradually 

 became vigorous and full-grown ; and were at length destroyed only after 

 repeated irruptions of mud and sand. 



6. The valleys in the carboniferous chain, near the longitudinal sections, 

 are not generally excavated on any lines of fault ; and are, in the severest 

 sense of the term, valleys of denudation. Yet the actual erosion on the 

 ledges of solid rock at the numerous waterfalls (of which I have attempted to 

 point out the origin), is in general so small as almost to demonstrate, that 

 there must have been a great change in the distribution of the water-channels 

 at some period, very recent when compared with that of the first elevation of 

 the carboniferous chain, before the deposit of the new red sandstone. 



* As an cKample of this, I may state that at Cross Pits, in the valley of Dent, the coal-seam 

 under the twelve-fathom limestone is divided by a band of clay, half an inch thick, into two parts, 

 with distinct mineral characters; and that the same coal-seam, with exactly the same subdivisions, 

 has been found in the mountain on the opposite side of the valley, at the distance of three or four 

 miles measured in a straight line. This seems to prove that a bed, not more than a fraction of an 

 inch thick, was originally continuous through an area, probably several miles in diameter. 



