evident, therefore, that this so-called fluidal structure may be explained as the result of movements imparted to a mass composed of solid crystals and a viscid base—and, on the other hand, that the perlitic spheroids must have been formed subsequently to the solidification of the entire mass.
The trichites are extremely minute and slender hair-like crystals, either straight or bent and twisted into most irregular curved and even zigzag forms. They are usually black and opaque, but when partially decomposed appear of a reddish-brown colour.
Spherulites also frequently occur in glassy rocks. They are globular in form, though quite distinct in character from the perlitic balls or spheroids, with which, however, they are not unfrequently associated. They are seldom very translucent, even in quite thin sections, but invariably polarize light, and exhibit a fine fibrous radial structure. They will be more fully described in the sequel.
These semicrystalline bodies were evidently the last substances formed prior to the solidification of the mass; for not only are the more perfectly crystallized constituents (felspar, mica, &c.) enclosed in them, but even the streams of microliths also occasionally pass straight through them—a fact which appears to have escaped the notice of previous observers. As regards the general order of formation, the evidence seems therefore to indicate that crystals of felspar, mica, quartz, &c. were enclosed in a viscous glassy magma, which was also crowded with innumerable microliths; before the mass solidified, the fluidal structure was imparted to it; and subsequently, during solidification, the radiating spherulites were formed, without disturbing the previous arrangement of the microliths[1].
The characters just described are extremely well shown by a single group of minerals in a section of one of the Kremnitz perlites (see fig. 7). Fibrous spherulites are here seen to be traversed by streams of microliths, and also to be penetrated by plates of mica and crystals of felspar. In the figure, one end of a plate of brown mica is enclosed in a spherulite, while the opposite end penetrates a crystal of orthoclase; the latter interferes with an adjacent spherulite, which is partly formed round it on the left side, and a partially included plate of mica projects from it on the right. It will also be seen that the streams of microliths invariably flow round all the crystals, whether large or small.
Having sketched the most prominent features of comparatively recent perlitic and spherulitic rocks, I will now describe a few of their ancient prototypes.
III. Structure of ancient Spherulitic Pitchstones and Perlites.
As previously observed, the principal rock-masses in the two parallel ridges near Wellington present the same general characters:
- ↑ I recently described the occurrence of an analogous case of crystallization without disturbance of previous texture, in one of the altered slates near Penzance. See Quart. Journ. Geol. Soc. vol. xxxii. p. 410.
