xxvi INTRODUCTION. 



p. xvii. It cannot be scratched with a file, but itself is hard enough to 

 scratch glass easily. It is infusible before the blowpipe ; is not acted on 

 by acids, whether cold or hot ; and, with the addition of carbonate of soda, 

 is easily dissolved to a glass before the blowpipe. None of the members of 

 the Quartz family have a specific gravity exceeding 2*84. 



Calc Spar or Calcite, the next mineral to Quartz as regards its abundance, 

 also assumes a variety of aspects. They may all be scratched with the point 

 of a knife ; they all effervesce on the addition of a drop of dilute muriatic 

 acid, and are infusible before the blowpipe, but shine with an intense light, 

 and are rendered alkaline by being converted into quick lime, when the 

 carbonic acid is expelled by heat. When not compact massive, the cleavage 

 is very perfect rhombohedral, and the specific gravity does not exceed 2"8. 



The Felspars rarely assume granular forms, and never occur fibrous or 

 columnar, but either in tabular crystals or in cleavable masses. They are 

 commonly colourless, or varying in tint from white to flesh-red, sometimes 

 bluish-green and brown, and have a vitreous lustre, which, in some instances, 

 inclines to pearly. The cleavage of Felspar is highly characteristic : one 

 face of cleavage is perfectly smooth, and another, nearly at right angles to 

 it, is somewhat less perfect. Orthoclase may be recognised from the other 

 varieties of Felspar, by having the two cleavage-planes at right angles to 

 each other. 



The Zeolites are most frequently associated together in cavities or cracks 

 in amygdaloidal rocks, though they are sometimes found in granite and other 

 rocks. The various members of the Stilbite group are distinguishable 

 by the pearly lustre of their cleavage. They are likewise remarkable for 

 often assuming laminato-radiated forms, and are frequently acicular or in 

 radiated masses consisting of slender fibres. 



Hornblende and Pyroxene are often not easily distinguishable when not in 

 crystals, except by chemical analysis. When crystallized. Hornblende often 

 occurs in six-sided prisms, while Pyroxene has commonly four-sided prisms. 

 They both vary in colour, from white to black through grass-green and 

 olive-green shades. Both are distinctly cleavable, with the exception of 

 Epidote, which has no very distinct cleavage. The crystals and the columnar 

 forms of the latter variety have also a more solid appearance, and present a 

 smoother surface ; and, when broken longitudinally, the prisms do not 

 show the cleavage-plane and that splintery look which are observable in 

 Pyroxene and Hornblende under similar circumstances. 



Micaceous Minerals consist of every thin and easily separable laminse. Of 

 these, Muscovite, Phlogopite, Biotite, and Lepidolite are closely related, and 

 possess in common the characters of having their lamina3 elastic ; of yielding 

 no water (or very little) in a tube ; of fusing only at the edges before the 

 blowpipe ; of not being acted on by acids ; and of affording, with a cobalt 

 solution, sometimes a clear blue, but generally a dull blue tint. The specific 

 gravity of this group varies from 275 to 3-3, and the hardness from 1-5 to 2. 



Pyrophyllite, 3Iarga7'ite, Evphyllite. — With the exception of the former 

 mineral, the laminae are rather brittle, and the colours are white, or of a pale 

 tint. They all afford water in a tube ; are fusible before the blowpipe at 

 their edges (Pyrophyllite swells out) ; afford little or no action with acids, 

 and give a blue colour with a cobalt solution. The specific gravity varies 



