CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 



Fig. 7. — Section of Crinoidal limestone, 

 from the Devonian (Hamilton Formation) 

 of Canada, enlarged ten times. The matrix 

 in which the Crinoidal fragments are en- 

 closed, is mostly a fine calcareous mud. 

 (Original.) 



fragments of Crinoids, showing that the materials of which they are 

 composed had been subjected to the action of the sea before being 

 consolidated into rock. In other 

 cases, especially among some of 

 the Crinoidal limestones of the Me- 

 sozoic period, the Crinoids seem to 

 have grown on the spot where the 

 limestone was deposited. Very 

 generally, the Crinoidal fragments 

 are sufficiently large and well pre- 

 served to be readily recognised, 

 even with the unassisted eye ; but 

 even when they have been greatly 

 abraded and worn down, their pres- 

 ence can usually be detected with- 

 out difficulty by an examination of 

 thin sections by means of the mi- 

 croscope. By this method it is, at 

 any rate, almost always possible to 

 determine whether or not a given 

 fragment is Echinodermal, since 

 the minute structure of the skele- 

 ton in the animals of this group is highly characteristic. 



None of the members of the great series of the Annulose Animals 

 can be said to play a very important part in the formation of lime- 

 stones. Apart from the occasional presence in limestones of the 

 calcareous cases of the Tubicolous Annelides, almost the only 

 Annulose animals which ever contribute to lime-making are the 

 Crustacea. In some cases, however, the calcareous crusts of certain 

 groups of Crustaceans (particularly the Trilobites and the Ostra- 

 codes) constitute a noteworthy element in the composition of lime- 

 stones. 



On the other hand, the two existing groups of the Molluscoids — 

 viz., the Polyzoa and the Brachiofioda, have both been extensively 

 concerned in lime-making. In many of the Palaeozoic limestones, the 

 remains of Polyzoa constitute a conspicuous feature, though they 

 cannot be said to form the bulk of the rock. In some of the 

 Secondary and Tertiary limestones, however, the rock is really made 

 up to a predominating extent of the calcareous skeletons of Poly- 

 zoa. Well-known examples of such so-called " Coralline limestones " 

 are found in the Upper White Chalk of the continent of Europe, 

 and in the " Coralline Crag " (Pliocene) of Suffolk and Norfolk. 

 The Brachiopods, again, exerted their greatest activity as lime- 

 makers during the Palaeozoic period, many of the limestones of the 

 Ordovician, Silurian, Devonian, and Carboniferous periods being 



