GREENSTONE. 



99 



character by which it can be distinguished from Primary. Green- 

 stone composed of felspar and hornblende, in which the felspar is 

 white, and sienitic greenstone, in which the felspar is red, sometimes 

 occur in beds among transition rocks, particularly of slate. But 

 more frequently rocks of greenstone, sometimes called Trap, occur 

 in an unconformable position, covering rocks both of the transition 

 and secondary class, and will be described in the chapter on Uncon- 

 formable Rocks ; after the description of coal strata, called by the 

 miners Coal measures. 



OBSERVATIONS ON CONFORMABLE TRANSITION ROCKS. 



The order of succession in conformable transition rocks is extremely 

 variable, and the thickness of the same beds differs greatly in differ- 

 ent situations. In one district we find a whole uninterrupted series 

 of calcareous strata, forming entire mountains ; and in an adjacent 

 district, the same series are widely separated by intervening beds of 

 slale, greywacke, or sandstone ; and many of the strata which occur 

 in one place, will often be wanting in another. We have before ob- 

 served, that calcareous transition strata are subject to sudden varia- 

 tions of quahty in the same mountain : we cannot therefore be sur- 

 prised, that in distant districts a great diversity should exist, both in 

 the number and thickness of calcareous strata of the same formation ; 

 no single stratum can be regarded as an universal formation. In 

 whatever manner the strata were deposited, the deposition has been 

 interrupted by causes to us unknown, which have accumulated thick 

 masses in one situation, and prevented their formation in other parts. 

 With respect to beds composed chiefly of the fragments of older 

 rocks, it is evident that the contiguity to rocks which were most easily 

 disintegrated, would produce thicker beds of fragments in certain sit- 

 uations than in others, and that their formation must be local. 



The organic remains found in transition rocks, belong almost ex- 

 clusively to genera no longer existing, and which do not occur in 

 the upper secondary strata. Vegetable remains are rare in transition 

 rocks; they occur sometimes in slate rocks. The trilobite is peculiar 

 to transition rocks : the gigantic species occurs in slate, and the 

 smaller species in limestone. The orthoceratite is chiefly found in 

 transition limestone ; univalve shells rarely occur in it. The prevail- 

 ing fossils in this class are madrepores, corailites, and encrinites. The 

 remains of vertebrated animals are rarely, if ever, found in transition 

 rocks. Many instances cited by foreign geologists of vertebrated an- 

 imals found in this class of rocks are erroneous ; the rocks in which 

 they occur belong to the secondary strata ; and it should be noticed, 

 that some English conchologists have described fossil remains from 

 specimens collected in particular counties, without knowing precisely 

 their true localities, or whether they were found in situ or in diluvial 

 deposits. In the near vicinity of the transition limestone in Derby- 

 shire, I have collected gryphites and nummulites, and even the fossils of 

 the chalk formation, but they had no relation to the ancient lime- 

 stone ; they were found in beds of graveL 



