374 PEOF. J. W. JrDD ox THE TERIIAET AXD 



Silica 38-006 



Alumina 0-286 



!Ferric oxide 2-933 



Perrous oxide 18-703 



ITanganous oxide 0-100 



Lime 0*336 



Magnesia 38-000 



Water 1-587 



99-945 



This analysis indicates a very ferriferous olivine appro aching in 

 composition to the variety known as hyalosiderite. 



The SjpinelUds. — Among the essential minerals of these rocks are 

 the minerals which have been grouped hy Fouque and Levy under 

 the convenient name of the spinellids, namely magnetite, chromite, 

 and picotite. These minerals, which are isomorphous, have now 

 been shown to pass into one another, by insensible gradations as the 

 proportion of chromic acid varies *. The first is opaque, the two 

 others more or less translucent and of a brown colour. Magne- 

 tite is the mineral which is usually found in the gabbros ; chromite 

 and picotite in the peridotites. 



It may be mentioned that, as has been proved by Dr. Hodgkinson, 

 of the I^ormal School of Science, all these rocks contain copper. 

 It is probable that nickel and cobalt are also present in varying 

 quantities, as well as chromium, manganese and iron ; but in 

 ordinary analyses no attempt is made to isolate the oxides of these 

 metals. 



Biotite — represented usually by a highly ferriferous and very 

 dichroic variety, is among the most common of the accessory minerals 

 of these rocks ; in some cases it becomes so abundant as to deserve 

 to be regarded as an essential constituent of the rock. 



Metallic Iron. — By the employment of Prof. Andrew's method, 

 Mr. J. T. Buchanan has succeeded in proving that some of the iron 

 in the gabbros of the Western Isles of Scotland is in a metallic 

 condition. 



§ 4. The Changes whics the Mikeeals or these Eoce:s have 



IWDEEGONE at GeEAT DePTHS EEOM THE StJEFACE. 



The great intrusive masses of the West of Scotland are of especial 

 interest to geologists, inasmuch as they afi'ord us an opportunity of 

 studying the conditions assumed by the same minerals under 

 varying conditions of depth and pressure. 



The intrusions of basic rocks in Mull, Ardnamurchan, Eum, and 

 Skye were originally of very different dimensions, those of Skye and 

 Ardnamurchan being the largest, that of Mull coming next in size, 



* Renard, Eep. Voyage H.M.S. ' Challenger,' vol. ii., Narrative, " On the 

 Petrology of the Eocks of St. Paul," p. 10 ; Wadsworth, Mem. Mus. Comp. 

 Zool. Harvard, vol. xi. (1884) p. 176. 



