132 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Peb. 5, 



velopment of the other, and, consequently, that the relationship of 

 the two classes of rocks is complemental. 



Fig. 2. — Section of the Carboniferous Rochs of Belgium and West- 

 phalia, showing the augmentation in the thickness of the " Sedimen- 

 tary " Strata, and the simultaneous thinning-out of the Carboni- 

 ferous Limestone towards the North-east. 



S.W. Belgium. The Eliine. Westphalia. N.E. 



%. 



1. Coal-measures. 



2. Millstone-grit (Flotz-leerer Sandstein). 



3. Carboniferous Limestone (absent in Westphalia). 



4. Lower Shales, expanding in Westphalia. 



* Devonian Schists, &c. at the base of the Carboniferous Rocks. 



(/.) I have abeady hinted at the cause of this inherent distinction, 

 hut it may be as well to state it in more precise language. As lime- 

 stones are by universal consent allowed to have resulted from the 

 exuvise of living animals, they will be accumulated in greatest 

 quantity wherever the conditions of life are most favourable. Now, 

 the fact that limestones, when they occur in considerable thick- 

 ness, are generally pure, and free from foreign matter, shows that 

 one of the first requisites for limestone-making animals is that they 

 should inhabit waters free from mud or sand. Where the White 

 Chalk is in greatest thickness, it is pure ; the same is the case with 

 the OoHte limestones, and with the Carboniferous Limestone of Derby- 

 shire which is of enormous thickness and contains very few beds of 

 shale ; but whenever these massive calcareous rocks begin to be 

 split up by the admixture of shales or sandstones, they become im- 

 poverished in mineral character and diminished in thickness*. The 

 formations in which these phenomena occur show us that the Mol- 

 lusca are to some decree independent of such changes, as the remains 

 of animals of this class are often abundant in sandy and muddy 

 deposits containing, small quantities of lime ; but, as a general state- 

 ment, it may be affirmed that clear and unpolluted water was essen- 

 tial to the full development of those delicately organized animals, the 

 Foraminifera, Zoophyta, Polyzoa, and Crinoidea, which are, and have 

 ever been, the most efficient elaborators of limestone rocks. 



It is almost superfluous to observe, that, in speaking of the neces- 

 sity of pure water to the full development of the marine animals 

 above named, carbonate of lime in solution is not understood as a 

 source of impurity. This mineral must necessarily be present as the 

 material from which the Zoophytes and other animals construct their 



* The Hmestones of the Culm of Devonshire, as compared with their repre- 

 sentatives at Bristol, are illustrations of this principle. At Bristol, where it 

 occurs in great force, the limestone is pure and crystalUne ; but in Devonshire, 

 where black shales are largely distributed amongst the beds of limestone, these 

 latter are frequently of so poor a quality that " even in the richer portions there 

 is seldom more than a third or fourth part which is actually burnt for lime." 

 See Memoir of Sedgwick and Murchison, in Geol. Trans. 2nd ser. vol. v. p. 674 



