OLDER PERIDOTITES OF SCOTLAI^D. 375 



while that of Rum is the smallest among the principal volcanic 

 centres. Of still smaller intrusive masses, however, we have 

 many examples, among which may be specially mentioned those of 

 Sarsta Beinn in Mull, and the Shiant Isles, north of ^kyo. 



These intrusive masses are fully exposed to our studj^, even their 

 central portions being laid bare by denudation ; and the results 

 obtained by a comparative study, by the aid of the microscope, of 

 examples derived from corresponding portions of masses of different 

 size and of different portions of same mass, are of very great 

 interest. 



If my interpretation of the geological structure of the district be 

 correct, it is a necessary inference that while the rocks of the CuchuUin 

 Hills of Skye about Loch Coruiskh, and those forming the western 

 extremity of Ardnamurchan, once existed at a great depth from the 

 surface of the volcanoes of which they formed a part, the similar rocks 

 forming the mountain masses of jMuU and Rum existed at a smaller 

 depth and under less pressure. Now I shall show that the several 

 minerals of these rocks, when they have formed parts of masses at 

 great depths from the original surface, often exhibit very striking 

 and suggestive differences in their characters from those which 

 have existed at smaller depths. 



It will further be demonstrated that a precisely similar series of 

 differences can be traced when the several minerals are followed 

 from the more superficial to the more profound portions of each 

 intrusive mass. 



"We shall describe minutely these changes in the case of each of 

 the minerals forming these rocks. In these Tertiary rocks the 

 question is fortunately not complicated or obscured by alterations 

 which have been set up by weathering action ; for, as a rule, the 

 minerals are strikingly fresh and unaltered. 



The Felspars. — The felspars of the more superficial portions of the 

 intrusive masses, and also those in the smaller intrusions and 

 apophyses, are usually remarkable for their strikingly clear and fresh 

 appearance. Under the highest powers of the microscope they are 

 seen to be traversed by many fine cracks, while a few cavities, some 

 of which contain liquids with moving bubbles, are scattered through 

 them. In such cases it may perhaps be inferred that the cavities 

 were formed during the original development of the crystal, and 

 that the cracks are due to the contraction of the mass during its 

 cooling from its original high temperature. 



In the felspars of rock-masses which were originally more deeply 

 seated, this perfect clearness seems to be quite lost. Cavities, 

 some of which contain liquid with spontaneously moving bubbles, 

 are present in enormous abundance. Prof. Zirkel has already re- 

 marked on the extraordinary abundance t)f such enclosures in the 

 felspars of the gabbros of Mull*. What is of especial interest 

 however, is the fact that these cavities in many cases are seen to lie 

 in fissures, or in bands parallel to fissures, in the crystal, and to 

 be connected by minute ramifying tubular processes. In many 

 * Zeitschr. d. deutsch. geol. Gesell, vol. xxiii. (1871) p. 59. 



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