162 
MELLO: MK KOSCOPICAL STRUC TURE OF ROC KS. 
known as tachylite.'^ Besides all the above-mentioned rocks, there is 
also an intermediate series, which frecjuently occur as intrusive 
masses or dykes, thus we have diorite, a rock composed principally of 
felspar and liornblende, also syenite ; porphyrite and phonolite are 
lavas which may be classed with these, j 
Turning now to the microscopical structure of these rocks, we 
frequently find in them a vitreous or glassy base enclosing crystalline 
minerals ; such a glass appears on a large scale in obsidian and 
the pitchstones and in tachylite. Now we have seen that a glass 
under the microscope presents no trace of structure, when the polar- 
iscope is used there is no double refraction observed, such as is 
exhibited hy all crystals except those of the cubic system. "Fel- 
spar may appear dark at the same time, but if the polarizer is 
rotated a few degrees and the prisms again crossed by moving the 
analyser, the felspar will transmit light." Thus a felsitic base, 
which at first glance might appear to be vitreous will show double 
refraction. When polarised it will break up as the prisms are rotated 
into variously coloured little particles, and is seen to be a granular 
compound of crystalline fragments, amongst which are a few more or 
less perfect crystals. Such a base is met with in felspar and in 
many other rocks. Again, the cpiartz of granite may at first sight be 
mistaken for a glassy base, as it is usually found without the angular 
form of crystals, having been the last mineral to crystallize, and 
therefore is modified by the others. It appears to be structureless, 
only containing numerous fluid and other cavities. But directly it is 
examined with polarised light its true character is revealed by the 
gorgeous dis])lay of colours it presents, broken into irregular patches, 
some of which show round their edges parallel wavy bands of colour, 
* Both in the Acid and in the Basic rocks a gradual passage mny be traced in 
Bome districts from hig-hly crystalline through less cryetaDine into glassy formp, 
and a gradation on the one hand has been shown to exist from granite through 
quar z fe'.nite, and rhyolite into obsidian, and on the other from gabbro into 
dolerite bat^ult and tachylite, such changes having resulted from the varying 
conditions under which the rocks had consolidated. (Judd, Q. J. G. S., yol. 
xlii., pt. 1). 
f In the case of these rocks also an insensible gradation has been traced from 
holo-crvstalline to glassy conditions, as from diorite into vitreous andesite. 
