100s" * GEOGNOSY. 3  [Boox 13 
sometimes be observed to be marked by clot-like patches or streaks 
of darker and lighter tint arranged in lines or eddy-like ‘curves — 
indicative of the flow of the original fluid mass. Rotated in the 
dark field of crossed Nicol prisms, such a natural glass remains 
dark, being perfectly inert in polarized light. It is therefore said to 
be isotropic, and may thus readily be distinguished from any enclosed. 
crystals which acting on the dight are anisotropic(p. 188). Perfectly 
homogeneous structureless glass without enclosures of any kind occurs 
for the most part only in limited patches, even in the most thoroughly 
vitreous rocks. Originally the structure of all glassy rocks at the 
time of most complete fusion may have been that of perfectly un- 
individualized glass. But as these masses tended towards a solid 
form, devitrification of their glass set in. Many forms of incipient 
or imperfect crystallization as well as perfect crystals were developed 
in the still fluid and moving mass, and were drawn out in the direc- 
tion of motion. In some cases so far has devitrification proceeded, 
that no trace remains of any glass. 
C, CrystaLiites.A—Under this name may be included minute 
ne 
ER 
ve CH 
29, i 

fia. 8.—AvcGiTeE CRYSTAL SURROUNDED BY MicrotirHs, FROM THE ViTREOUS BASALT 
oF EskpALE. Murr, MAGNIFIED 800 DrameTErRs, 
inorganic bodies possessing a more or less definite form, but generally 
without the geometrical characters of crystals. They occur most 
commonly in rocks which have been formed from igneous fusion, but 
are found also in others which have resulted from or have been 
altered by aqueous solutions. They seem to be early or peculiar 
forms of crystallization developed in artificial slags, and in many 
vitreous rocks, under conditions not yet well understood. The 
' This word was first used by Sir James Hall to denote the lithoid substance obtained 
by him after fusing and then slowly cooling various “ whinstones” or volcanic rocks. 
Sinee its reyival in lithology it has been applied to the minuter bodies above de- 
seribed, and a distinction has been drawn between crystallites and microliths. It seems 
to me most convenient to retain the term erystallites as the general designation of all the 
indefinitely erystalline or incipient forms of individualization among minerals, and to sub- 
divide these by the employment of such names as Vogelsang’s Globulites, Longulites, 
Microliths, &c, ‘The student should consult this author’s Philosophie der Geologie, 
p. 189; Krystalliten, Bonn, 8vo. 1875; also his deseriptions in Archives Neéerlandatses 
vy. 1870, vi. 1871. Sorby, Brit. Assoc. 1850, 

