
114-0 - GEOGNOSY. = > [Boox Il, — 
are peculiarly rich in minerals,-—mica, garnet, tremolite, actinolite, : 
anthophyllite, zoisite, vesuvianite, and many other species occurring 
there often in great abundance. Many varieties of colour and ~ 
texture occur among these limestones, as may be seen in the 
numerous kinds of ornamental marble. 
Marble is regarded by most geologists as a metamorphic rock, 
that is, one in which the calcium carbonate, whether derived from an — 
orgauic or inorganic source, has been entirely recrystallized in situ. — 
In the course of this change the original clay sand or other 
impurities of the rock have been also crystallized, and now appear 
as the crystalline silicates just referred to. Marble occurs in beds 
and large lenticular masses associated with crystalline schists on 
many different geological horizons. In Canada it occurs of Lauren- 
tian; in Scotland of Lower Silurian; in Utah of Upper Carbon- 
iferous; in Southern Hurope of Jurassic age. 


SS] MOST, 
Fic. 16.—Microscorio Srructoure oF WHITE STATUARY MARBLE, 
Maaniriep 50 DIAMETERS. 
Dolomite (Magnesian Limestone) consists typically of a yellow 
or white crystalline massive aggregate of the mineral dolomite; but 
the relative proportions of the calcium and magnesium carbonates 
vary indefinitely, so that every gradation can be found, from pure lime- 
stone without magnesium carbonate up to pure dolomite containing 
45°65 per cent. of that carbonate. Ferrous carbonate is also of common 
occurrence in this rock. The texture of dolomite is usually distinctly 
crystalline, the individual crystals being occasionally so loosely held 
together that the rock readily crumbles into a crystalline sand. A 
fissured cavernous structure is of common occurrence; eyen in 
compact varieties cellular spaces occur lined with crystallized dolomite 
(Itauchwacke), the crystals of which are often hollow and sometimes 
enclose a kernel of calcite. Other varieties are built up of spherical, 
botryoidal and irregularly-shaped concretionary masses, Dolomite 
in its more typical forms is distinguishable from limestone by its 
greater hardness (3°5—4:5), higher specific gravity (2°8—2-95), and 
