Composition^ and Analogies of Rocks, 205 



islands ; though the accessoiy causes, arising from the decompo- 

 sition of previous limestones, must be admitted, as far as regards 

 the secondary strata. But the mere existence of primary lime- 

 stones thus operating by their destruction to assist in producing 

 new ones, is not itself a proof that these are original and inde- 

 pendent of animal sources. The existence of animal remains in 

 primary schists has just been mentioned ; and I have elsewhere 

 described one instance in which these occur in a limestone situated 

 beneath gneiss. Thus far they might have contributed to the 

 production of even the primary limestones ; and if they are not 

 more frequently found among them, causes for that are not 

 •wanting. 



In the first place, primary limestones are not only comparatively 

 rare, but geologists, having adopted the hypothetical opinion that 

 they ought not to contain animal remains, make it a rule, inva- 

 riably, to rank sucli instances among these transition series ; 

 without thinking it necessary to investigate the subject by the 

 rigid rules of pure geological analysis ; by position, and relation 

 towards the neighbouring strata. It is further obvious, that the 

 primary rocks have undergone great disturbance, and, in many 

 instances, serious changes ; and, even among the secondary strata, 

 it is known that, in such cases, the animal remains are often obli- 

 terated. The fusibility of limestone has been demonstrated ; and 

 it has often been sho\vn tliat many of the primary strata bear 

 marks, scarcely to be disputed, of the action of long-continued 

 heat. Thus it is to be expected, that their organic remains, if 

 they ever existed, should have been obliterated ; and if this has 

 not happened in the case just quoted, of shells under gneiss, it 

 is because the bed in which they are immediately situated, is, 

 in fact, a quartz rock included in the limestone. If any confir- 

 mation of the plausibility of this view were required, it is found 

 most distinctly detailed by nature in Sky, and in the Isle of Man. 

 Where the conchiferous beds are actually converted into pure 

 crystalline limestone by the action of the incumbent trap, it is 

 undistinguishable from the primary rocks of the same kind, and 

 all the shells have disappeared ; while, in some parts of the gra- 



