66 PKOF. J. W. JTJDD ON TEKTIAKY 



wliicli petrographers have been in the habit, perhaps not very wisely, 

 of giving such names as " viridite," " opacite," and " ferrite," and 

 which can sometimes be detected assuming crystalline forms, though 

 the species of the minerals may be very difficult to define. Lastly, 

 the amorphous glass may take up water and assume the characters 

 of palagonite. 



VI. Peopoetions op the seveeal Mineeals m the Kocks. 



Among the more highly crystalline types of these rocks, there is 

 found the greatest diversity in the proportions of the several minerals 

 which constitute them, I need not point out how greatly this 

 will affect the bulk-analyses of such rocks ; such analyses, indeed, 

 must be almost useless in the case of very highly crystalline types, 

 unless the analyst has selected in the field a specimen which 

 appears to be of average mineral constitution, or has been able to 

 " sample," by some means or other, a number of different specimens f 



Professor Zirkel, from his examination of the Mull gabbros, was 

 led to the conclusion that the three constituent minerals exist in 

 the following proportions in these rocks : — plagioclase felspar 3, 

 olivine 2, diallage 1 *. A general examination of the rocks over 

 the whole area leads me rather to conclude that while felspar is 

 usually the predominant constituent, the pyroxenes are more abun- 

 dant thafli the olivine. But within a very small area — and this is 

 notably the case in Ardnamurchan and Rum — we may find examples 

 of these rocks containing the most diverse proportions of the three 

 constituent minerals. Some varieties from Ardnamurchan are made 

 up to the extent of at least three fourths of their mass of felspar, 

 while in others the felspar is the least abundant of the three con- 

 stituents (see Plate lY. figs. 3 & 4). 



This inequality in the relative proportions of the constituents is 

 exhibited in an exaggerated manner in the " contemporaneous " 

 veins and inclusions ; these are often binary compounds of the 

 essential minerals of the gabbros, or form varieties in which one 

 or the other constituent is present in excessive proportion. 



It has been already pointed out how, by the disappearance of 

 felspar, the gabbro passes into picrite, by the disappearance of 

 augite, into troctolite, and by the disappearance of olivine, into 

 eucrite ; while by the excessive development of accessory consti- 

 tuents like enstatite and picotite fresh varieties such as Iherzolite 

 may arise. 



YII. Stetjctctees of the Eocks. 



It is not necessary to recapitulate the evidence that the gabbros, 

 the dolerites, the basalts, and the tachylytes do not present any 

 essential difference in their bulk-analysis f ; that is to say, their 

 chemical composition varies within the same comparatively narrow 

 limits ; and there is nothing therefore in their ultimate constitution 

 to forbid the idea that all of these rocks may have consolidated from 

 magmas containing the same chemical elements in identical pro- 

 portions. 



* Zeitschr. d. d. Geol. Ges. Bd. xxiii. (1871) p. 69. 



t Quart. Journ. Geol. Soc. vol. xxx. (1874) pp. 233-236. 



