MINERALS AND GEOLOGY OF CANADA, 439 
mentioned under this division, consult Pary Y. Certain siliceous 
rocks, called “tripoli’”? and ‘infusorial marls,’”’ are formed entirely 
of the tests of diatoms and other infusoria. (See Part IV.) 
Clay Slates are merely consolidated clays. They have a fissile 
structure, and are chiefly of a grey, greenish, brown, or black colour. 
Clays are also of various colours, as white, greenish, yellowish, bluish, 
black, and red. Those which contain little or no iron, burn white, 
and yield consequently white bricks. Many clays are highly calcare- 
ous; others, bituminous, &c. Note.—The term shale is often ap- 
plied to fissile consolidated clays; but this term is applied equally to 
fissile or slaty limestones and sandstones. When the term is used, 
therefore, the kind of shale should also be signified: as an argillace- 
ous shale, an arenaceous shale, and so forth. Bituminous shales, as 
regards their mineral base, may be also arenaceous, calcareous, &e. 
Limestones and Dolomites are principally, perhaps, of chemical 
formation. Water containing free carbonic acid (derived from decay- 
ing vegetable matters, &c.) dissolves a certain amount of carbonate 
of lime, but the bicarbonate, thus formed, is easily decomposed by 
various natural agencies, even by mere exposure to the atmosphere, 
and a precipitation of calcareous matter takes place. In this manner, 
calcareous tufas (so common in many of our swamps, streams, &c.), 
together with stalactites and stalagmites, are produced; and similar 
processes, acting on a larger scale, may have given rise to extensive 
depositions of limestone strata in ancient seas and lakes. Some lime- 
stones, again, are formed almost wholly of the calcareous shells or 
tests of crinoids, foraminifera, and other organisms (see Parr IV.) ; 
but others are, undoubtedly, mechanical or rock deposits, derived 
from the wasting of coral reefs and older limestone formations. 
Limestones consist of carbonate of lime, more or less pure; dolo- 
mites, of carbonate of lime and carbonate of magnesia in equal 
atomic proportions; and dolomitic limestones of these two carbo- 
nates in other proportions, the lime carbonate generally predomina- 
ting. Dolomites and dolomitic limestones appear in many cases to 
have been simple chemical precipitates, and, in others, to haye origin- 
ated from the alteration of limestone rocks by the action of soluble 
magnesian salts. These calcareous rocks are of various colours: 
grey, white, black, yellowish, &c. Their texture is sometimes very 
close and uniform. At other times, the stone is made up of small 
spherical concretions, when the texture is said to be “ oolitic.”? Oolitic 
