io6 ARAGONITE AND CALCITE FOSSILS. 



regarded as separated from the water in which the carbonate of lime 

 was originally dissolved. The distinction into fresh- water and marine 

 limestones is not of much importance, though the fresh-water strata are 

 probably more largely due to the precipitation of lime by the growth 

 of plants, like Chara, than to the agency of mollusca and other kinds 

 of animal life, which form so large a part of marine deposits. The 

 carbonate of lime of limestone sometimes exists in the crystalline form 

 of calcite, sometimes in the form of aragonite, and many shells have one 

 layer of calcite, and the other layer of aragonite. There is no means 

 known by which calcite can be changed into aragonite, the former being 

 a remarkably stable substance, but aragonite is as strikingly unstable. 

 When its temperature is raised it passes into a mass of crystals of cal- 

 cite ; it is also easily dissolved, and since calcite is usually deposited 

 from cold solutions of carbonate of lime, it happens that organisms 

 formed of aragonite are often removed entirely from a deposit, or 

 replaced by structureless calcite. This difference explains not only 

 the circumstance of preservation of many groups of fossils, but also 

 important points in the general structure of limestones. 1 The follow- 

 ing table may be useful in explaining the chemical condition of the 

 shelly skeletons of the chief groups of fossils : 



ARAGONITE ORGANISMS. CALCITE ORGANISMS. 



Cephalopoda are aragonite. I Brachiopoda are all calcite. 



Gasteropoda are mostly aragonite ; but ! Annelida are calcite. 



Patella, fusus, I/itorina, Purpura, 

 have the outer layer calcite. 

 Laniellibranchida are mostly aragonite, 

 but Ostrea and Pecten calcite ; Pinna, 

 MytiLus, Spondylus, outer layer calcite. 



Crustacea are calcite. 



Echinodermata are calcite. 



Polyzoa are a mixture of calcite and 



aragonite. 

 Alcyonaria, calcareous forms are calcite. 



Hydrozoa millepora, mainly aragonite. I Foraminifera, calcite. 

 Actinozoa, almost wholly aragonite. j Corallines, mainly calcite. 



All marine limestones are formed from the remains of these 

 organisms, or by chemical precipitation and deposition. 



Of late years it has been demonstrated that everywhere through- 

 out the deep ocean a white limestone is accumulating which, when 

 dried, presents the consistency and appearance of white chalk. In the 

 Atlantic this white globigerina ooze, as it is termed, consists almost 

 entirely of shells of such Foraminifera as Globigerina, Pulvinulina, and 

 Orbulina, mostly entire, with some otolites of fishes, and the mutilated 

 dead shells of half-a-dozen genera of pteropods. With these are asso- 

 ciated a small proportion of finer material which consists of the struc- 

 tures termed coccoliths and rhabdoliths, 2 with a few spines and 

 skeletons of the silicious radiolarians, and fragments of spiculas of 

 sponges. Besides the surface forms of Foraminifera, Cristellarian and 

 Milioline 3 forms occur which lived among the ooze, together with 

 sponges, corals, starfishes, the higher invertebrata, and a few fishes. 

 Below the surface layer thus composed is a somewhat firmer layer, 



1 Sorby, Brit. Assoc. Reports, 1862, Sect., p. 95. 



2 Wyville Thomson, " Challenger " Voyage in the Atlantic. 



3 Carpenter's Foraminifera, Ray Society, 1862. 



