OF THE AVESTEEX ISLES OF SCOTLAND. 



351 



presGDted by the f'erro-magnesian silicates, that led von Richthofeii 

 and Zirkel to insist on the analogies of these rocks with plutonic 

 rocks, which were, at the time when these authors wrote, generally 

 believed by Continental petrographcrs to be of Pre-Tertiary age. Any 

 glass-inclusions that the felspar-crystals may once have contained 

 have, of course, like the glass in the matrix, undergone devitrifi- 

 cation, while, as Zirkel has shown in similar rocks in j^orth Ame- 

 rica, bands of cavities containing liquids are abundant. This 

 character is also conspicuously exhibited by the quartz, which either 

 as a primary or secondary constituent is not unfrequently found in 

 these rocks. I have already stated the grounds that lead me to 

 conclude that these bands of liquid-inclusions are of secondary 

 origin. In many cases, however, the felspar-crystals, while pre- 

 serving their outward form, have been completely transformed, and 

 now only exist as pseudomorphs. The minerals which replace the 

 original substance of the plagioclase felspars are pistacite, zoisite, 

 and other minerals of the epidote group, with some new felspars of 

 secondary origin. Occasionally quartz and calcite have resulted 

 from the extreme alteration which the felspars have undergone. 



The ferro-magnesian constituents of these rocks, — whether pyro- 

 xenes, amphiboles, or micas, — are almost always in a more or less 

 altered state. A fibrous structure and a green colour have been deve- 

 loped in them, and, as this change goes on, curious modifications of 

 the optical properties of the minerals are brought about. Frequently 

 we find, as the result of this action, green isotropic materials are 

 formed to which the name of " Yiridite " may be properly applied. 

 But, in most instances, we find that from the products of the decom- 

 position of the original ferro-magnesian silicates, various minerals 

 of the chlorite group are developed, and crystallize out from tlie 

 mass. A still further change is marked by the destruction of these 

 chlorites, and the formation at their expense of various epidotes, 

 among which the varieties known as pistacite and a lime-epidote with 

 a little manganese are conspicuous. 



The pyroxenes of these rocks are usually the monoclinic forms 

 or augites ; the rhombic enstatites, though not unfrequently pre- 

 sent, being, as in the case of the associated basic rocks of this 

 district, usually subordinate and seldom, if ever, a predominating 

 constituent of the rocks. The amphiboles were probably horn- 

 blendes, long acicular and tufted forms abounding. It is clear that 

 in the unaltered rocks these crystals of araphibole and those of 

 biotite were surrounded by zones (resorption-halos) composed of 

 pyroxene and magnetite grains, the latter mineral only remaining 

 in the altered state of the rock. Many of the rocks contained biotite 

 as an original constituent ; but I shall show in the sequel that much 

 biotite has been developed in these rocks as a secondary constituent. 



That titanofcrrite was often present in these rocks, as well as 

 magnetite, is shown by the forms of the opaque crystals and by the 

 way in which, as the result of alteration, they become surrounded 

 by the dense white product and the colourless substance derived 

 from it, known as " leucoxene." Sometimes the abundant magne- 



