GABBKOS ETC. IN SCOTLAND AND lEELAND. 83 



the negative crystals are decomposition-products — mixtures of 

 various hydrated oxides. 



In the most deeply seated gabbros, like those of Skye, Ardna- 

 murchan, and the Carlingford mountains, the form of augite known 

 as pseudo-hypersthene abounds. This is a variety in which the 

 schillerizing process has been set up along two or three mutually 

 intersecting planes. In gabbros of less deeply seated origin, as on 

 the outskirts of the larger masses, and in the centres of those of 

 smaller dimensions, as in Mull, E-um, and St. Kilda, the form of 

 augite with schillerization along one plane only, which is known as 

 diallage, abounds ; and there is every gradation from this diallage 

 into ordinary augite as we examine portions that have consolidated 

 nearer the surface (see Plate lY. figs. 5, 6). 



There are precisely the same relations between the rocks containing 

 the forms of enstatite known as hypersthene and bronzite respectively. 



§ 3. Action of Steam and other Gases at the Surface, 

 In a district which has been subjected to such an enormous 

 amount of denudation as that which we are considering, it can 

 scarcely be expected that many of those chemical changes of the 

 surfaces of lavas, which are so familiar to all students of volcanic 

 districts, should be exhibited. There is indeed distinct evidence that 

 in many cases the surfaces of the old lavas had weathered into a soil, 

 and . sometimes that they had suffered considerable denudation 

 before they were covered up by the later flows. Consequently the pre- 

 servation of the surface-products formed by the action of the escaping 

 steam and other gases upon one another and upon the materials of 

 the lavas through which they escape must be of very rare and ex- 

 ceptional occurrence. 



Nevertheless, traces of this kind of action do occur in the district 

 we are describing. In some of the basalts the vitreous matter of 

 the ground-mass is seen to be converted into the hydrated glass 

 which has received the name of palagonite. Such basalts may be 

 conveniently termed " palagonite-basalts," and examples of them 

 occur in Mull and some other of the Western Isles of Scotland. 



The infilling of the cavities formed by the escape of bubbles of 

 gas in the basaltic lavas, whereby a scoriaceous basalt is converted 

 into an amygdaloid, is not improbably due, in part at least, to the 

 chemical actions set up by the steam and other gases escaping 

 through the lavas on their way to the surface. It is a very note- 

 worthy circumstance in connexion with these rocks, that where the 

 amygdaloidal kernels abound, the mass of the rock has undergone 

 the greatest amount of decomposition. Thus, in a lava in which the 

 central and compact portion shows but little signs of decomposition, 

 except in the serpentinization of the olivines and a partial altera- 

 tion of the felspars, the scoriaceous upper and under portions 

 will have undergone the most complete alteration into a soft 

 crumbling mass. The collection of liquids and gases in these cavi- 

 ties has converted them into laboratories of synthetic mineralogy. 

 The several minerals and glasses of the surrounding rock have 



g2 



