[C1. 41], Sedgwick Collection, from near the top of Bwlch Mawr, is a rock of greenish-grey colour, with rather fewer felspars than the last. In the microscope the ground is seen to be largely occupied by squarish crystals of felspar, between which is some crypto-crystal-line matter. The larger felspars seem all plagioclase, but they are all much decomposed, some of the smaller ones are perhaps ortho-clase. There is no pyroxenic mineral preserved; calcite aggregations result, and also viridite extinguishing in no position. There are many minute quartz grains in the ground, which have the appearance of being secondary. Apatite is abundant; there are also black and brown crystals of iron oxides (ilmenite). A few crystals of epidote have been formed. No hornblende is to be found, and amorphous matter in the ground is uncertain, yet the rock may most conveniently be placed among the porphyrites. It has not, however, the andesitic structure of the Carn Boduan rock. The dark colour would be due to the chloritic matter and iron oxide scattered through the ground.