86 ANNUAL REPORT 
lime per unit volume, but requiring also more heat in the burning. The 
rocks are liable to be contaminated with varying smaller amounts of im- 
purities and especially may be associated with the carbonate of another 
alkaline earth, magnesia. But as a rule the clayey impurities are small in. 
amount. On breaking the rocks show a splintery or conchoidal fracture. 
The specific gravity varies from 2.6 to 2.8. When treated with cold hydro- 
chloric acid it dissolves readily, except in the presence of a larger amount 
of magnesium carbonate; the clayey matter is practically all left behind. 
No fixed limits can be drawn for the composition of these rocks; it is 
bound to vary, depending on the conditions which prevailed during the 
formation period. If precipitated in muddy water, the limestone is certain 
to contain clay; if thrown down in concentrated aqueous solutions, as in | 
inland lakes or bays of the sea, other salts like the magnesium compounds 
are bound to crystallize out with the lime. 
Dense, Dull, Non-Crystalline Limestone.—These are principally lime 
deposits produced in muddy water, and hence are high in clay. It is quite 
evident that the variety of composition of these materials must be wide, 
since the impurities and the clayey substances may represent almost any 
possible combination of materials. As the clayey constituents increase 
the limestone changes to cement rock and finally merges into a calcareous 
shale. The rocks of this division are, as a rule, very hard and dense and 
possess a dark blue or dark gray color. [Frequently they show shale, 
structure, caused by mountain pressure. The calcium carbonate is very 
often associated with magnesium carbonate, so that the rock is really a 
clayey dolomite. Carbonaceous matter is frequently deposited in these — 
rocks, from graphitic carbon to bitumen and volatile hydrocarbons. Some 
of the so-called “stink stones” owe their fetid odor to petroleum com- 
pounds or to hydrogen sulphide. Limestones of this character are often 
high in iron sulphide (pyrites), which accelerates the dlosonmpoe tle of the 
rock by weathering, spoiling it for building purposes. 
To the dull, hard limestone divisions belong also the rocks having a 
concretionary or oolitic structure. 
_ Many sedimentary rocks show a coneretionary structure. This may 
be part of the original sedimentation or may be due to subsequent segre- 
gation around a center.* This structure is produced by the collection of 
mineral matter, say calcium carbonate, from the surrounding rock or 
from without around some center so as to form a nodule, large or small, 
as the case may be. If these nodules are close together they may form a 
practical source of lime, but this is usually not the case. Oolite consists 
of more or less round grains of calcium carbonate, each grain having 
been deposited in concentric shells around some center. The oolitic rocks 
have the appearance of a mass of fish eggs, as, for instance, the well-known 
Indiana oolitic stone. | 
*Geikie, Text Book of Geol gy, page 510. 
