154 A POPULAR EXPOSITION OF THE 
masses, (limestone, marble, &c.,) is perhaps 
the most abundantly distributed of all minerals, 
quartz only excepted. In Canada, in the erys- 
talline limestones of the Laurentian Series, and 
in the vast calcareous deposits of the Huronian, 6 
Silurian, and Devonian formations, it occupies Mime 
extended areas, although much concealed by 
the overlying clays and gravels of the Drift. e 
Rhombohedrons, scalenohedrons, and other 
crystals are frequently met with in cracks and 
hollows in these limestone and other rocks.* 
Stalactitic masses are also found under similar 
conditions ; and nodular concretions occur in 
the amygdaloidal traps of Lake Huron and Lake Superior. Fine 
erystallizations, also, amongst the copper deposits of these lakes. 
White and variously coloured marbles of much beauty are obtained 
from our Laurentian rocks and from the more modern metamorphic 
series, south of the St. Lawrence; but these, with the other economic 
limestones of Canada, will come under review in,Parr VY. of this 
Essay. It should be observed, however, that many of our so-called 
limestones are dolomites or dolomitic limestones, containing magnesia. 
See under Dolomite below. 
SOY, 
Fig. 47. 
Nore.—Carbonate of lime is a dimorphous substance, oceurring under two dis- 
tinet series of crystal-forms: the crystallographic difference being accompanied, 
moreover, by a difference of hardness and other physical characters. It thus 
forms two distinct minerals: Cale spar and Arragonite. Whilst the former, or 
normal condition of carbonate of lime, is exceedingly abundant, the latter is 
comparatively rare. Arragonite crystallises in rhombic prisms and other trimetri¢ 
combinations (the compounds of which often present a pseudo-hexagonal aspect), 
and also in fibrous, coralloidal and botryoidal masses. Small splinters, when 
heated, become immediately opaque, and crumble or decrepitate gently into 
powder, a peculiarity by which this mineral may be distinguished from eale spar. 
Fibrous arragonite appears to occur sparingly amongst the Lake Superior traps, 
and occasionally in thin coatings on the sides of cracks in some of our limestone 
rocks, but nowhere in very distinct specimens. 
* Whilst writing this description, for example, we have received some large crystals (com- 
binations of a rhombohedron and two scalenohedrons) from a cavity in the Trenton limestone 
(Lower Silurian Series) of Huntingdon township, in the county of Hastings,C.W. The 
cavity contained an immense number of these crystals. 
