Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/297

 size of a walnut, the surfaces of which are generally covered with oxide of iron, probably arising from a partial decomposition. This is the case, not merely near the surface, but in some degree even where the rock has been quarried to a considerable depth. This peculiarity renders it very difficult to obtain such a fracture as shews the real nature of the rock, and makes it almost impossible to procure good cabinet specimens.

§9. When I first began to examine the rocks of which these hills are composed, I was particularly struck with the great variety that presented itself, for almost every specimen which I detached within a very limited space, offered a new character. A closer examination, however, shewed that there is a greater uniformity than I at first suspected, and that the diversity of appearance depends on the different proportions in which the same materials are united together. Felspar, hornblende, quartz, and mica, forming different compound rocks, and varying as much in the size as in the proportions of the ingredients, constitute the greater part of the range. There are very few rocks in which the size of the component parts is so minute as to give the internal structure a homogeneous appearance.

§ 10. If every “ compound granular aggregated rock, composed of felspar, quartz, and mica,” is to be considered as granite, a very great part of the Malvern hills is composed of it; but among the various compounds of that nature, found in this place, there are very few which present the same appearance as the granite of Alpine countries; they have not the decided crystalline structure, which these granites usually exhibit; nor are the several parts so closely intermixed. The felspar is generally red, and predominates considerably in the mass; sometimes the quartz and sometimes the mica is wanting, but more