THE LOWER-SILURIAN DISTRICT OF SHROPSHIRE. 453 



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 subse- 

 quently, during solidification, the radiating spherulites were formed, 

 without disturbing the previous arrangement of the microliths*. 



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. 



