LA TOUCIIE — FORMATION OF LIMESTONE BANDS. 



21 



ciency of that substance, is a fact which I have not seen hitherto 

 satisfactorily accounted for, but of which we may hope to find some 

 rational explanation. 



The surface of the country in Shropshire, where these rocks 

 abound, may be roughly described as consisting of ridges of hills, of 

 Which one flank is steep, the other shelving; the crest of the ridge 

 being composed of a band of limestone, which, by its hardness, has 

 evidently resisted the action of the denuding forces which have worn 

 away the softer strata, and thus have formed the valleys. 



I. Now, while the proportion of lime to earthy matter is immensely 

 great in these comparatively thin bands which form the crests of these 

 hills, it is as remarkably deficient in the rest of the strata; in the 

 one case, the fossils, and their bed also, are often found to be a mass 

 of carbonate of lime, while, in the other, not only the substance in 

 which the fossils are deposited is deficient in that substance, but the 

 fossils themselves have frequently lost the lime which once, beyond 

 all doubt, entered into their composition. 



II. Besides these remarkable and extensive layers of limestone, we 

 meet occasionally with minor bands of more limited extent, and in 

 some places with nodules of limestone varying from an inch or two 

 to eighty feet in diameter. 



Such, briefly, are the facts of the case, and attempts have been 

 made to account for them in the following manner : — 1st. It has 

 been supposed that occasionally, during the deposition of the strata, 

 a sudden but transitory development of carbonate of lime took 

 place in the waters of the primaeval seas — an hypothesis which seems 

 to be at once refuted by the existence of the nodules I have alluded 

 to : and, 2ndly, a more plausible theory, that these bands have been 

 formed in a manner analogous to coral reefs ; that the animals of 

 which the fossils are the remains, secreted vast quantities of lime ; 

 that an accumulation of that substance took place around them, and 

 so ultimately a layer of limestone was formed. 



The last hypothesis seems to me also unsatisfactory, for while it 

 may account for those cases where fossils are enormously abundant, 

 as in some parts of the Wenlock limestone, it seems to fail in those 

 where they are absent ; and such instances are frequent. We find 

 large masses where fossils are rare, and yet that they have not been 

 destroyed or absorbed in any way is proved by the existence of occa- 

 sional specimens in a very perfect state. Witness such fossils as the 

 lllccmis Barriensis ; the Euomphalus, and other shells at Woolhope, 

 preserved perfectly in the midst of an enormous bed of limestone. 

 And does this hypothesis give any satisfactory account of the nodular 

 masses ? Can they be supposed to be formed in the same way as 

 coral reefs ? 



Altogether, the facts above mentioned seem to me to point to 

 some kind of aggregation of particles, like to like, probably after the 

 deposition of these strata. AVhile they were in the transition state 

 between mud and rock, the limestone particles which had been 

 equably distributed throughout them arranged themselves in definite 



