Jan., 1890 . crystallization in rocks. 17 
called, are of more basic composition than the later ones. 
In a few cases a further separation has been effected of the 
constituents of the rock, and the part which last consolidated 
(in these cases as a glass) has been analysed, with the result 
of showing that it contains a considerably larger percentage 
of silica than the average of the rock. For instance, in the 
case of a dyke examined by Mr. I’anson, as noticed by Mr. 
Teall, Q. J. G. S., 1884, p. 225, the general analysis of the 
rock showing about 58 per cent, of silica; the glassy base 
contained 70. I have also called attention to somewhat 
similar relations in the case of certain so-called segregation 
or contemporaneous veins in the rocks of Penmaenmawr 
and Rowley. Mr. Teall has further suggested (Geol. Mag., 
Mar., 1885) that certain of the quartz felsites of the Cheviot 
district may be represented as, so to speak, the mother liquor 
out of which some of the more basic porphyrites, &c., have 
been separated. 
Individual crystals frequently show by the arrangement of 
the glass fragments they have enclosed that the rate and 
regularity of the growth of the crystal has varied. Thus we 
frequently find the central parts of a crystal pretty free from 
iucluded masses, while the outer zones contain many or vice 
versa. Occasionally the inclusions are arranged in several 
zones alternating with zones which are almost perfectly 
clear. In one slide in my collection this is repeated six or 
seven times. 
Another slide has an arrangement which does not come 
under this head. The basalt which is used as road metal in 
Sydney contains porphyritic crystals of felspar of consider¬ 
able size. One of these, furnishing a section of about Jin. 
long and ^in. broad, is divided into two very distinct halves, 
one of which swarms with inclusions of brown glass of 
elongated shape lying parallel to the sides of the section, 
while the other half of the section is almost free from any 
inclusion at all. 
If now we inquire as to the processes which initiate the 
formation of crystals, we shall find the best materials for 
study among those rocks which have a portion, whether larger 
or smaller, in the condition of glass. Ijn such a glassy rock 
we frequently find minute rounded bodies which appear to be 
the primary effort towards crystallization, although they 
show no action on polarized light. To these bodies Vogelsang, 
who studied the subject both in natural and artificial products, 
gave the name of globulites. These globulites are often 
found combined together in strings, either retaining their 
individual shapes or sometimes combined to minute cylindrical 
rods which have received the name of longulites. 
