DESCRIPTIONS OF ROCKS. ALT 
Weisselbere (from Rosenbusch), in which the microlites are 
distinetly crystalline in form, and some give evidence that 
they are feldspar crystals, others that they are angite and 
magnetite, and indicate that the rock is intermediate be- 
tween a glass and a doleryte. Thus there is a passage to 
ordinary stone. ‘Trap or doleryte has been used for making 
bottle-glass ; and attempts have been made to manufacture 
glass directly from a variety of granite containing little 
quartz. 
Hruptive rocks, that have come up through fissures. 
often have glassy particles among the stony in the part 
near the walls of the fissure when not so through the inte- 
rior of the mass; and many such rocks, covering large areas, 
have glassy grains among the stony grains, or a glassy mag- 
ma, because the cooling ‘eencrally was not slow ‘cnowgh for 
complete lapidification : ; or they have an undcfined base 
when examined in thin slices, which the microscope does 
not resolve into crystalline grains. Such portions of a rock 
are described as wnindividualized. An wnindividualized 
base exists in the basalt of Truckee Valley, the character of 
a slice from which, highly magnified, is given in fig. 7, from 
Zirkel; feldspar crystals, of their usual rectangular forms 
(part of them sanidin). one of the largish crystals of chryso- 
lite, and smaller irregularly-shaped augites, are imbedded in 
-a base which consists of a glass-like substance ; and in this 
material there are extremely small globulite or: ains which 
are globules of devitrified glass or incipient crystals. ‘The 
glassy unindividualized base occupies the spaces among the 
crystalline portions. 
These differences in crystalline texture are of small im- 
portance compared with differences In mineral and chcmi- 
cal composition. They are results of accidents, and, at 
the best, lead only to a distinction of varieties among kinds 
of rocks. The presence of a little glass, or of disseminated 
large crystals in a porphyritic way, does not make the rock 
essentially different in kind. If, however, the glassy na- 
ture is manifest in the external appearance of the mass, it 
is convenient to call the rock by a separate name. 
Porphyritic rocks are sometimes named as if porphyry 
was a distinet kind of rock, or as if the porphyritic section 
of a kind of rock merited special prominence. But, as re- 
cognized beyond, ““felsyte- porphyry ” is porphyritic felsyte ; 
‘‘dioryte-porphyry” is porphyritic dioryte ; ‘ diabase-por- 
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