' . 



84 



of augite-picrite (augite-peridotite. WILLIAMS) from Montrose Point yielded 

 only 15'34 per cent, of magnesia, whereas the lowest of the unaltered picrites 

 of TSCHERMAK yielded 23'34 per cent. (1) The altered picrites contained 

 much lower per-centages of magnesia ; but then in these rocks there is direct 

 evidence of the removal of magnesia, for the olivines are represented by 

 pseudomorphs of calcium carbonate (sometimes over 40 per cent.), and a basic 

 silicate containing lime, magnesia, alumina and alkalies. 



With regard to the nomenclature of those rocks belonging to group A., 

 which do not contain olivine there is little to be said. If we except the terms 

 hornblendite and pyroxenite proposed by Professor DANA, special names 

 have not yet been applied to members of this class. The only rocks of 

 this class with which the present writer is acquainted in Britain occur in 

 the Hebridean gneissic system of Sutherland ; and they may possibily form 

 an integral portion of that system. They have, however, a massive habit 

 and are composed of hornblende, an intensely pleochroic pyroxene 

 (amblystegite, JUDD) and biotite. They contain no trace of either olivine 

 or felspar. 



It is interesting to note that the rocks of Group A. are mostly 

 granitic in texture. Limburgite and augitite are merely varieties of basalt in 

 which the felspathic constituent has not individualised itself. In bulk 

 analysis they agree with the basalts and not with the rocks of Group A. 



CHARACTERS OF THE ROCK-FORMING MINERALS. 



Olivine. The most important mineral of this group of rocks is olivine. In 

 the typical peridotites the Iherzolites, saxonites and dunites it forms more than 

 half of the entire mass of the rock. In dunite it forms, together with a small 

 quantity of chrome iron ore, the whole ; so that the rock itself yields an 

 analysis agreeing substantially with that of the mineral. In the Iherzolite of 

 Lake Lherz it forms, according to DAMOUR, two-thirds of the entire mass. 

 It occurs usually in the form of crystalline grains ; sometimes, however, it 

 shows an approach to definite crystalline form. As a rule it is nearly 

 colourless in thin section, but sometimes it appears markedly yellow or 

 yellowish-brown. As this is the colour of the iron-olivine (fayalite) of the 

 eulysite of Tunaberg, it seems reasonable to infer that such varieties are 

 rich in iron. (2) 



Dr. OEBBEKE (3) has isolated and analysed the olivine of the picrite 

 (palseopicrite) of the Schwarze Stein, Nassau ; and thus proved that it contains 

 a considerable amount of lime. It is intermediate in composition between 

 normal olivine and the monticellite of Vesuvius. 



(1) Porphyrgesteine Osterreichs, p. 244. 



(2) See JUDD, Q.J.G.S., vol. XLL, p. 



(3) Ein Beitrag znr Kenntniss des Palaeopikrits. "Wurzburg 1877. Lime-olivipe of the 

 Schwarze Stein, SiO a = 42'53, MgO. = 35-68, CaO = 14-08, FeO = 6-48. 



