as exhibited in the Devonian Limestones of Devonshire. 21 



in thickness. Having prepared and examined several hundreds, 

 inchiding those of every geological period, I am convinced that, 

 mth the exception of concretions, and such as have undergone 

 chemical metamorphism, nearly all may be described as organic 

 sands or clays. In the same manner that felspar can be broken 

 up into grains and formed into sand, or decompose into fine 

 granules of clay, by the removal of its alkali, so can calcareous 

 organic bodies be broken up into fragments, or decay into fine 

 granules, by the abstraction of the organic matter that binds 

 them together. It is thus that, in the neighbourhood of coral 

 reefs, and in localities such as some of the West India Islands, 

 there are deposits of both these characters, either alone or more 

 or less mixed together, which may be most satisfactorily described 

 as organic sands, organic clays, or organic sandy clays. Of pre- 

 cisely similar nature are our older Bx-itish limestones, differing 

 only in the kind and proportion of the various organisms, and in 

 the amount of subsequent chemical change ; and I shall there- 

 fore adopt this nomenclature in my descriptions of them. 



The ultimate organic structure of the fragments is often so 

 perfectly preserved in limestones, that there is no difficulty in 

 deciding from what kind of organism they have been derived, 

 and the mineral constituents may also be distinguished by their 

 action on polarized light and other peculiarities. In order to 

 know the relative bulk of the constituents, whether organic or 

 mineral, I select a portion of the object that shov/s the general 

 character in a satisfactory manner, and draw on an evenly thick 

 piece of drawing-paper the outline of the various portions with 

 a camera lucida, distinguishing by some mark the diiferent con- 

 stituents. I then weigh the whole in accurate balances, and 

 afterwards cut out the various portions and weigh them. In 

 this manner their actual proportions may be ascertained with 

 greater precision than in most cases is necessary. Of course the 

 difference between this kind o? physical analysis and a chemical 

 is very great, and very different information is afforded, as will 

 be seen by inspecting those given further on in this paper ; and 

 it must be borne in mind that one gives the relative volumes, 

 and the other the relative weights. Eoth are most valuable for 

 particular purposes; but, for the majority of limestones, the 

 physical method is very much more useful in geological inves- 

 tigations. 



In most limestones, even when no chemical change has oc- 

 curred, the calcareous matter derived from decayed organisms 

 has become more or less crystalline in ciystals of varying dimen- 

 sions. This is often well seen in organic clays. There has also 

 generally been introduced by infiltration a considerable amount 

 of calcareous spar, that has filled up the cavities existing amongst 



