Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/541

Rh the object alternately to right and left, the previously well-defined granules gradually assume shadowy indefinite forms of variable dimensions, alternately appearing and disappearing; while the perlitic curved lines are seen to pass continuously through them, and even the minute belonites and trichites appear with opposite ends enclosed in two of the adjacent pseudo-granules.

When the devitrification appears to be complete, and a granular mosaic structure the most distinct, there is not the slightest appearance of any such fragmental character when examined in ordinary light. Under all powers of the microscope the matrix then seems to be a perfect glass traversed by straight or tortuous streams of microliths and intersected by the perlitic fissures already described.

Although these investigations show conclusively that certain molecular changes have taken place, it should not be overlooked that the structure resulting from devitrification differs in character from that of a true felsite; and I think the rocks here described afford no evidence in favour of the view held by Vogelsang and others, that the base of the so-called quartz-porphyries may have been originally of a glassy nature.

The thin sections are frequently traversed by narrow veins, which pass through all the constituents, as shown in fig. 10. A fine fissure has here cut through the matrix, a spherulite, and an enclosed crystal of felspar. In all cases these veins appear to consist of the same substance as the devitrified matrix; and when the latter is penetrated by one of them, it is impossible to distinguish the one from the other except in certain positions of the prisms, when the continuity of the vein may still be detected.

Quartz-veins are not uncommon; and in some parts of the spherulitic rock chalcedonic nodules are rather abundant.

Although the microscopical structure of these rocks is in itself decisive as to their origin, I may add that their chemical composition is also in perfect accordance with their other characters. Mr. J. A. Phillips kindly made for me an analysis in duplicate of a specimen of the devitrified perlitic pitchstone, with the results shown in the following Table, in which I have also included analyses of two Miocene perlites:—