Those rocks which contain an excessive proportion of the bases, especially magnesia and ferrous oxide, and are therefore composed largely of unisilicates, may be conveniently classed as ultra-basic rocks. Such rocks constitute a small but highly interesting group, which is characterized as follows:—They have a very low percentage of silica, ranging generally from 35 to 45, with a high specific gravity, varying from 3 to 4; while the ferro-magnesian silicates, olivine and enstatite, enter largely into their constitution. Felspar is often altogether absent from these rocks, and, when present, appears to be always represented by the basic species anorthite, or one approaching in composition to that type. Many of these ultra-basic rocks may be very conveniently grouped, as Professor Rosenbusch proposes, under the name of “peridotites;” rocks, that is, in which the unisilicate, olivine, forms the prevailing constituent. There are other ultra-basic rocks, however, which contain a considerable proportion of the bisilicates, and these form a link between true peridotites and the ordinary basic rocks. Among these may be noticed the picrites, in which olivine is united with a considerable proportion of augite, hornblende, or biotite, anorthite-augite rock (eucrite), anorthite-hornblende rock (corsite), and anorthite-olivine rock (troctolite or forellenstein). The peridotites and their allies are of especial interest to geologists on four different grounds : First , their mode of occurrence has been thought to be suggestive of their having come from deeply seated portions of the earth's crust, and they have therefore been supposed to afford some indications concerning the constitution of