CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 25 



water Molluscs as by those which inhabit the sea. The recent 

 " shell-marls " are examples of deposits of this nature, and various 

 Tertiary and Secondary limestones are more or less extensively 

 charged with the shells of fresh-water Gastropods and Bivalves. 



Finally, it is to be noted that animals are not the exclusive agents 

 concerned in the building up of limestones. Certain of the calcare- 

 ous Algae — such as the " Corallines," the " Nullipores," and the 

 singular family of the Dactyloporidce — are capable, singly or in com- 

 bination with other organisms, of forming accumulations of lime, 

 sometimes upon a most extensive scale. The two latter groups, in 

 particular, have given rise to vast masses of limestone. Examples 

 of the Dactyloporidce occur even in the Palaeozoic limestones, but 

 the most famous and most extensive deposit formed by Algce of this 

 group is the well-known " Gyroporella-limestone " of the Bavarian 

 and Tyrolese Alps, the age of which is Triassic. Limestones formed 

 more or less largely of " Nullipores " (Lithothamnio?i) occur to some 

 extent in the Secondary rocks, and are extensively developed in the 

 Tertiary series. The most famous of these is the " Nulliporen- 

 kalk " or " Leitha-kalk " of the Vienna basin (fig. 9), which attains 

 a considerable thickness, and extends from Austria, through the 

 Balkans, to xAsia Minor and Persia. 



In connection with the subject of the constitution of the ordinary or- 

 ganic limestones, the researches of Dr Sorby on the precise chemical 

 composition of calcareous organisms demand a brief notice. The car- 

 bonate of lime in calcareous organisms exists sometimes in the condition 

 of aragonite, sometimes in that of calcite. The chief differences between 

 these two allotropic conditions of calcium carbonate are : (1) that calcite 

 is optically uniaxial, whereas aragonite is biaxial ; (2) that calcite has a 

 specific gravity of about 2.72, whereas the density of aragonite is 2.93 ; 

 and (3) that aragonite is harder than calcite, as shown by the fact that 

 the former will scratch a crystal of Iceland spar along the line of the 

 short diagonal of one of the crystalline facets, whereas the latter will not 

 do so. In the second place, the composition of the skeleton of calcareous 

 organisms varies in different groups, some having a skeleton wholly of 

 calcite and others wholly of aragonite, while in some cases the skeleton 

 is composed in part of calcite and in part of aragonite. The following- 

 table shows the principal variations in this respect, as determined by the 

 researches of Sorby :— 



1. Foramhrifera. — The test of the calcareous Forami?iifera appears to 

 be in general composed of calcite, though a certain amount of aragonite 

 seems to be sometimes present. [In the porcellanous Foraminifera the 

 test is very probably wholly composed of aragonite.] 



2. Madreporaria. — The true Corals have the skeleton composed, 

 mainly or wholly, of aragonite. 



3. Alcyo?iaria. — The skeleton of the Alcyonarian Corals is mainly of 

 calcite, but with indications of the presence of a small amount of aragon- 

 ite or phosphate of lime. 



4. Fchinodermata. — The skeleton is always composed essentially of 

 calcite. 



