Theoretical Considerations. 285 



Vegetable impressions have been found in considerable quantity at 

 the excavations for coal in Wrentham. The most common is what I 

 have supposed to be several species of Calamites, of Ad.< Brongniart. 

 Some of them are several inches in width ; and are marked out on 

 the rock by a plaited layer of green indurated talc a most remark- 

 able mineralizer ! Plate XIII. fig. 41, represents a small portion of 

 one of those Calamites, which exhibits numerous small seams run- 

 ning obliquely across the specimen, like the C. nodosus of Brongni- 

 art. On fig. 42 may be seen two quite different impressions : a and 

 b are furrowed longitudinally, and appear to be branched : perhaps a 

 part of an Equisetum. The other fan-like impression, with radiated 

 stria, may perhaps belong to the genus Cyclopteris of Ad. Brongni- 

 art : though I am not without suspicion that this also may be an 

 Equisetum. 



Theoretical Considerations. 



In general the theoretical views that have been presented in rela- 

 tion to the origin of the new red sandstone, will apply to the gray- 

 wacke. Two or three circumstances only, in relation to this latter 

 rock, need any additional remarks. 



One is the more decided evidence, which the graywacke presents, 

 of the operation of chemical agencies in its production. This is ob- 

 vious in the more crystalline aspect of the rock in general, and espe- 

 cially of certain varieties; and in the numerous veins traversing this 

 rock, which must have resulted from a play of chemical affinities. 

 It may be difficult, in the present state of geological science, to assign 

 any satisfactory reason, why the older rocks, with some exceptions, are 

 more highly crystalline than the newer. But if it be admitted that in- 

 ternal heat in the earth, which every thing proves must once have been 

 very powerful, has been gradually operating less and less upon the crust 

 of the globe, why is it not a natural inference, that the older the rock 

 the more crystalline would be its structure: that is, if we admit that the 

 heat has been great enough to change the arrangement of the particles 

 of rocks, whose origin was mechanical : and it appears that such a 

 change may take place, to some extent at least, far below a melting 

 heat. Only admit then, that the graywacke is an older rock than the 

 new red sandstone, (and it seems to me that the veins in the former 

 are sufficient to prove this,) and we should expect in it a more chem- 

 ical structure. 



Another peculiarity in the graywacke, so far as we are acquainted 



