LA TOUCnE — rOKMATION OF LIMESTONE BANDS. 
21 
ciency of tbat substance, is a fact which I have not seen hitherto 
eatistactorily accoanted 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 shelviug; 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 limeatone 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 prinijeval 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 fiiil in those 
wliere 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 
lllcenus Barriensis ; the Euomijhalus, 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. While they were in the transition state 
between mud and rock, the limestone particles which had been 
equably distributed throughout them arranged themselves in definite 
