171 



direction, it will be seen that any method of treatment not based on broad 

 geological facts must be, to a great extent, artificial. The only way in which 

 we can produce a cut and dried system of classification a so-called logical 

 system is by shutting ourselves up in our laboratories and ignoring 

 transitional forms. 



In what follows we shall, for convenience of description, consider first 

 of all the coarsely crystalline rocks with granitic texture ; and afterwards 

 the medium grained, finely crystalline, compact and vitreous rocks of similar 

 composition. The former usually occiir as large intrusive masses, but 

 sometimes also as dykes, sheets and veins ; the latter occur as dykes, sheets, 

 veins and lava flows. The vitreous condition is comparatively rare amongst 

 basic eruptive rocks, and when it does occur is usually seen at the margins of 

 dykes and, less frequently, of lava flows. 



The term gabbro Avill be used as a family name for the granitic 

 modification of the basic magma. (1) It is thus made to include a great 

 variety of mineral aggregates. On the one hand we have the varieties due 

 to the occurrence of the original minerals plagioclase, augite, enstatite and 

 olivine in varying proportions ; on the other hand we have the varieties 

 due to the more or less complete alteration of the original constituents into 

 secondary products, such as saussurite and hornblende. Troctolite, eucrite 

 and norite thus become varieties of the great gabbro-family, depending on 

 variations in the relative proportions of the original minerals ; euphotide 

 (Hauy) becomes a variety dependent upon the alteration of the original 

 constituents into saussurite and hornblende. 



Rocks of the gabbro-family are extensively developed on the 

 west coast of Scotland, in the islands of Mull, Skye and Rum, and 

 on the mainland in the peninsula of Ardnamurchan. They were 

 named hypersthenite, by Maccullock, under the impression that 

 the principal ferro-maguesiaii constituent was hypersthene. (2) Precisely 

 similar rocks occur in the Carlingford district of Ireland, north of Dundalk 

 Bay. The Scotch rocks have been studied microscopically by Professor 

 ZIRKEL (3) and Professor Jrnn, (4) and the Irish rocks by Professor YON 

 LASAULX. (5) These rocks are of great interest in consequence of their 

 geological age. The definite recognition of the fact that they belong to the 

 Tertiary period has destroyed the illusion that all gabbros are necessarily 

 pre-tertiary ; and, at the same time, greatly discredited the view now rapidly 

 disappearing from the minds of geologists, that petrographical characters can 



(1) It will be extended also to such rocks as those of Mosedale, near Carrock Fell, which 

 contain quartz in addition to the augite or diallage and plagioclase. These may be termed 

 quartz-gabbros. They are not typical basic rocks, but represent the transition from basic to 

 intermediate rocks. They are the plutonic representatives of the andesitic dolerites. The 

 Mosedale rock contains 56 p.c. of silica. 



(2) Western Isles of Scotland. 1819. 



(3) Geologische Shizzen von der Westkuste Schottlands. Z.D.G.G. ' Vol. XXIII.. 1871, 



p. 1. 



(4) Q.J.G.S. Vols. XLI. and XLII. 



(o) T.M.M. NeueFolge. Vol. I., 1878, p. 427, 



