

ae - @EOGNOSY. ~ ‘[Boox I 
see that the glassy base has been clarified round the larger in- 
dividnals by the abstraction of the crowded smaller microliths. 
With the crystallites may be grouped the characteristic amorphous 
or indefinitely granular and fibrous or scaly matter which constitutes 
the microscopic base in which the definite crystals of felsites and por- 
phyries are imbedded (pp. 104,135). The true nature of this substance 
is not yet understood. Between crossed Nicol prisms it sometimes 
behaves isotropically, like a glass, but in other cases allows a mottled 
glimmering light to pass through. It is a product of devitrification | 
where, though the vitreous character has disappeared, its place has 
not been taken by recognizable crystals or crystalline particles.’ 
Every gradation in the relative abundance of crystallites may be 
traced. In some obsidians and other vitreous rocks, portions of the 
glass can be obtained with comparatively few of them; but in the 
same rocks we may not infrequently observe adjacent parts where 
they have been so largely developed as to usurp.the place of the 
original glass, and give the rock in consequence a lithoid aspect 
, 441). 
© D. Read ae rocks are composed of the detritus of pre- 
existing materials. In the great majority of cases this can be readily 
detected, even with the naked eye. But where the texture of such 
detrital or fragmental (clastic) rocks becomes exceedingly fine, their 
true nature may require elucidation with the microscope. An 
obvious distinction can be drawn between a mass of compact detritus 
and a crystalline or vitreous rock. The detrital materials are found 
to consist of variously and irregularly shaped grains with more or less 
of an amorphous and generally granular paste. In some cases the 
grains are broken and angular, in others they are rounded or water- 
worn (p. 154). They may consist of minerals (quartz, chert, felspars, 
mica, &c.), or of rocks (slate, limestone, basalt, &c.), or of the remains 
of plants or animals (spores of lycopods, fragments of shells, erinoids, 
&c.). It is evident therefore that though some of them may be 
crystalline, the rock of which they now form part is a non-crystalline 
compound. Where water containing carbonate of lime or other 
mineral matter in solution has permeated a detrital rock, it has 
sometimes allowed its dissolved materials to crystallize among the 
interstices of the detritus. But this change does not conceal the 
fundamentally secondary or derivative nature of the mass, 
2. Microscopic Structures of Rocks. 
_We have next to consider the manner in which the foregoing 
microscropic elements are associated in rocks. ‘This inquiry brings 
before us the minute structure of rocks, and throws great light upon 
their origin and history.? 
F oa aot ome Beschaf. p. 280. Rosenbusch, vol. ii. p. 60. 
The first broad classification of the microscopic structure of rocks was that 
pored by Zirkel, which, with slight modification, is h p } pipe 
iene 1. 6s g is here adopted. Mik, Beschaff. p. 265. 
