170 
MELLO: MICROSCOPICAL STKUC'TUKE OF ROCKS. 
It has been noted tliat " the igneous rocks of the Tertiary and 
recent periods, as a general rule, exhibit far less perfectly crystalline 
structures than those which belong to the older formations. Again, 
the older the rock is, the gTeater the probability that its mineral 
constituents will have undergone alteration, sometimes this will be 
so gTeat that hardly one will remain in its original form. Augite 
may be converted into hornblende, olivine and enstatite into serpen- 
tine, and so on. Also many rock forming minerals may be made," as 
Professor J. W. Judd has pointed out,''' "to assume new and unfamiliar 
aspects by the developement of enclosures along certain planes 
within their crj^stals." Thus some pyi'oxenic minerals are seen to 
contain numerous enclosures in the form of thin jjlates and rods 
arranged along parallel planes in the crystals, these, when the crystals 
are looked at in certain positions, give it a peculiar semi-metallic 
lustre or "chatoyant" character, which is termed schiilerization, thus 
diallage and liypersthene are shown to be schillerized forms of augite 
and enstatite, and Professor Judd observes that " the production of 
the schillerized condition is related to the depth at which the crystals 
have originally existed ; schillerized forms being found only in deep- 
seated intrusive masses. The solvent action of heated water and 
other fluids acting under great pressure attacks the crystal along 
certain planes, not always cleavage planes, but of chemical weakness, 
negative crystals are thus formed which become tilled with the 
products of decomposition." 
In the older igneous rocks of Palaeozoic age it not unfrequently 
happens that the whole of the original minerals will have been con- 
verted into pseudomorphs, partly by deep-seated action, partly by 
atmospheric agencies, and often as the rock is traced from within 
outwards, the alteration will be found to increase. Where extreme 
change has taken place it will sometimes be possible to infer that a 
given mineral has once formed part of a rock, by the characteristic 
enclosures which are found in the alteration products. 
Another point which has been made out is that there are distinct 
petrogi-aphical provinces within which the rocks erupted during any 
* Q. J. Q. S., vol. xli., pi. 3. 
