OLDER PERID0TITE3 OF SCOTLAND. 369 



Maskelyne, * amblystegite by Yom Rath f, and a similar niineral 

 from Mont Dore by Descloiseaux J. 



The minerals found in an unaltered form at these different 

 localities were at first regarded as being excessively rare. But in 

 1879 Prof. Fouque showed that the lavas of Santorin contain in 

 great abundance an unaltered rhombic pyroxene which he referred 

 to hypersthene§. In 1883, Mr. Whitman Cross demonstrated the 

 existence and wide distribution of a large and important class of 

 lavas, distinguished by the presence in them of a rhombic pyroxene 

 which he also referred to hypersthene jj . In the same year Mr. TeaU 

 and Dr. Petersen pointed out that certain British rocks from the 

 Cheviot Hills contain the same rhombic pyroxene 1[, and these, 

 as shown by the former author, are of especial interest as being 

 of pre-Tertiary age. Subsequent observations have shown these 

 rhombic pyroxenes to be among the most widely distributed of the 

 rock-forming minerals. Much attention was directed to them as 

 constituting an important constituent of the pumice thrown out by 

 Krakatoa during its last great eruption ; and they have been found 

 in the lavas of South America, Java and Sumatra, Japan, the 

 Philippine Islands, and many other volcanic districts. 



Nor are the rhombic pyroxenes less common among the plutonic 

 than among the volcanic rocks. In the Schemnitz district the Ter- 

 tiary diorites and quartz-diorites ** frequently contain a considerable 

 proportion of the rhombic pyroxene, which, as in the associated 

 lavas, has been generally confounded with the hornblende. Teller 

 and Yon John have described an interesting series of rocks of the 

 same class from the Tyrolft, and the diorite of Penmaenmawr has 

 been shown by Rosenbusch and others to be an enstatite-diabase:J::|:. 



The rhombic pyroxenes are bisilicates of magnesia and iron 

 (MgO, EeO) SiOg, in which the relative proportions of magnesia and 

 iron may vary to almost any extent. Lime is often almost wholly 

 absent, but appears to be capable of replacing, perhaps to a limited 

 extent, the other bases. 



It is well known that the colour, the physical properties, and the 

 optical constants of varieties of the augites, the micas, and other 

 similar groups, are modified in the most remarkable manner by the 

 quantity of ferrous oxide or other bases in their composition. But 

 in the group of the rhombic pyroxenes the variations in character 

 dependent on composition are of the most extreme kind. 



At one end of the series we have enstatite proper, a colourless 

 mineral without trace of dichroism ; but as the magnesia is replaced 



* Trans. Eoy. Soc. vol. clx. p. 204. t Poggend. Ann. (1869), cxxxviii. 531. 

 J Manuel de Mineralogie, tome. ii. (1874) p, xviii. 

 § Santorin et ses Eruptions (Paris : 1879). 

 II Bull. U. S. Geol. Survey, no. i. 



% Geol._ Mag. Dec. ii. vol. x. (1883) p. 346. J. Petersen, Mikroskopische 

 »und chemische Untersuchungen am Enstatit-Porphyrit aus den Cheviot Hills. 

 (Kiel, 1884). 



** See Quart. Journ. Geol. Soc. vol. xxxii. (1876) pp. 292-325. 

 tt Jahrb. k-k. geol. Eeichsanst. vol. xxxii. (1882) pp. 589-584. 

 II Die massigen Gesteine, p. 352. 



