404 Prof. T. G. Bonney. On the [Nov. 27, 



little lenticular patches, Home containing more or less of quartz and of 

 a white mica, but in most the space is mainly occupied by two minerals 

 the one (probably the dark specks visible in the hand specimen) 

 occurs in rather rounded grains, which are darkened by a dusty 

 opacite and by minute specks of a yellowish mineral. It sometimes 

 appears to exhibit a slightly " radial " structure, but often this is not 

 distinguishable. It is not isotropic, though it only gives dull colours 

 with the Nicols. Occasionally it appears to be bordered with small 

 prisms of the mineral next to be described. This occurs in prisms of 

 rather " bladed " aspect, commonly, as it seems, four-sided, often 

 about '02 or '025 inch long, and about one-fourth to one-eighth the 

 breadth. There is a clearly marked, though not frequent, basal 

 cleavage, making a large angle with the sides. The mineral is a very 

 pale greenish-yellow in colour, rather granular in aspect, contains occa- 

 sionally enclosures of opacite, <fec., and gives moderately bright colours 

 with the Nicols. Though extinction takes place at a rather small angle 

 with the sides of the prism, the mineral is olearly neither nniaxial nor 

 orthorhombic. It is not unlike an epidote, but I do not think it belongs 

 to this group ; moreover, there is nothing in the macroscopic aspect 

 of the rock to suggest the presence of epidote, the glimmering scaly 

 mica and the rather lustrous spots of a black mineral being all that is 

 visible with a strong lens. I suspect that these minerals are rather 

 nearly related, and both belong to the group of anhydrous alumina 

 silicates varieties, perhaps, of fibrolite or kyanite. Unfortunately my 

 collection is not well provided with specimens containing these non- 

 alkaline alumina silicates, so that I am unable to give a more precise 

 definition. The rock, however, evidently belongs to the *' frucht- 

 schiefer " group, and I should conjecture has come from rather high 

 up in a metamorphic series, probably occupying a position above 

 those already described. 



It is then evident that, as Mr. Whymper asserts, Sara-urcu has no 

 claim to be called a volcanic mountain. All the specimens which he 

 has brought from it are metamorphic rocks. They do not, indeed, 

 belong to the earliest types, such as the coarse gneisses of the Hebrides, 

 but still they are greatly altered. Unless there were very clear 

 evidence to the contrary, I should regard them as members of the 

 Archaean series.* Hnmbolt (" Cosmos," vol. v, p. 44, Bonn's edit.) 

 mentions the occurrence of " greenish-white mica-slate with garnets," 

 at the foot of a " black trachyte," at Penipe, not far from the mouth 

 of the Bio Blanco ; further on, " at the hacienda of Guansce, near the 

 shore of the Rio Puela," and probably below that schist, a granite of 



It will be remembered that in Bolivia and Peru the eastern (and here much the 

 more elevated) chain of the Andes consists of sedimentary (silurian) rocks, the 

 western (ill-marked) range being volcanic. See D. Forbes, " Quart. Jour. Geol. Soc.," 

 vol. xvii, p. 7. 



