GABBROS ETC. IN SCOTLAND AND IRELAND. 89 



It has not been found necessary to give representations of the 

 porphyritic varieties of all these types, though such exist. Nor did 

 it seem desirable to complicate the question by introducing the more 

 highly altered forms, though these arc o^ considerable interest as 

 illustrating the fact that the degree of alteration is not necessarily 

 a mark of great geological age. 



Plate IY. 



grabbros passing into dolerites. 



All the figures on this Plate are magnified V-- 



Fig. 1. Slightly altered gabbro (" olivine-gabbro ") of Ai-flnamurchan. This 

 may serve as an adiiiirable example of the most perfectly bolocrystalline 

 type of these rocks, when the minerals exhibit only the incipient stages 

 of change. The felspar is seen in the section forming broad plates 

 with much polysynthetic twinning, and a few irregular cracks and 

 bands of inclusions. The brown augite shows the development of 

 enclosures along lines of strain and fissure, but only the first trace of 

 the foliation characteristic of diallage. The olivine which from its 

 colour would apiDcar to be highly ferriferous, is perfectly clear and 

 free from serpentinization, but exhibits the development of secondary 

 magnetite aloiig the cracks, with the first traces of schillerization along 

 planes parallel to the optic axis of the crystal. There is no original 

 magnetite in the rock. 



Fig. 2. More altered gabbro (" olivine-gabbro ") of Loch Ooruiskh, Isle of Skye. 

 In this rock the changes due to deep-seated action (schillerization) 

 have gone a little further. The broad plates of felspar, though per- 

 fectly clear, contain more liquid and solid enclosures than in the last; 

 the pale brown augite-crystnls are, in their exterior portions, converted 

 into diallage, by schillerization along planes parallel with the ortho- 

 pinacoid; the olivine is also more altered than in the last example, 

 and in some cases is rendered completely opaque by secondary mag- 

 netite. The augite in the rock is to some extent replaced by a ferri- 

 ferous enstatite, with bronzite structure partially developed in it. 



Fig, 3. Gabbro passing into eucrile (anorthite-augite rock) from Carlingford 

 Mountain, Co. Down, Ireland. In this rock, the broad plates of felspar 

 are seen to have undergone a considerable amount of deep-seated 

 alteration. The pale-green augite is considerably changed along the 

 lines of strain and fissure. The rock contains but little olivine (none 

 appearing in the drawing), so that, the felspar being anorthite, it ap- 

 proximates to eucrite. There is no original magnetite in the rock. 



Fig. 4. Gabbro, passing into troctolite (anorthite-olivine rock), from the West 

 Glen, St. Kilda, near the junction of the gabbro with the granite. The 

 crystals of felspar are not so large and broad as in the previous ex- 

 amples, and in some cases are seen to be enclosed in the augite, so 

 that thei-e is an approach to the ophitic structure ; they are perfectly 

 unweathered, but show traces of schillerization. The augite is small 

 in quantity and in some parts of the rock wholly wanting ; it is scbil- 

 lerized on one set of planes and so takes the form of diallage. The 

 olivine is abundant, darkened by secondary magnetite along the cracks, 

 and also forms scattered grains or groups of grains, in the midst of 

 the felspar-crystals. 



[Figs. 3 and 4 illustrate the less normal types of these rocks which result from 

 variations in the proportions of the constituent minerals.] 



Fig. 5. Altered gabbro ("olivine-gabbro") from Mam Clackdig, Isle of Mull. 

 The felspar (labradorite) of this rock is traversed by many bands of 

 cavities containing both liquid and solid materials. The augite is 

 convei'ted, by schillerization, into very typical diallage ; the relations 

 of the augite and felspar crystals in some cases indicate an approxi- 



