GABBROS ETC. IN SCOTLAND AND lEELAND. 87 



last mentioned is reconstructive. The effect of schillerization and 

 weathering is to bring about a destruction of the original crystals 

 and an admixture of their materials. But under the influence of 

 great crushing movements rearrangement and recrystallization is 

 brought about, the new minerals thus formed arranging themselves 

 in more or less distinct and parallel folia, and the rock becoming 

 a schist. A very instructive example of this action has lately been 

 described by Mr. Teall as occurring in a dyke at Scourie *. But in 

 the district we are now describing, where no such great movements 

 have taken place since the formation of the rocks, changes of this 

 kind are not exemplified. 



XII. SUMMAET OF RESTnDTS. 



The igneous rocks of Iceland, the Faroe Islands, the Western 

 Isles of Scotland, and the northern part of Ireland, present a striking 

 similarity in character, and belong to the same petrographical 

 province. Among the salient characters which distinguish the basic 

 rocks of this province may be mentioned the prevalence of the ophitic 

 structure, as well as the absence of porphyritic crystals of augite or 

 hornblende, and of enclosures of olivine, enstatite, and augite. 



That the whole of these rocks were erupted during the Tertiary 

 period, there cannot be the smallest doubt. They overlie, unconfor- 

 mably, the youngest members of the Cretaceous series, and are 

 interbedded with strata containing a very characteristic Tertiary 

 flora. 



The basic rocks of this area described in the present memoir 

 offer a beautiful example of the transition of varieties with a 

 highly crystalline structure, through every intermediate form, into 

 the vitreous type. The holocrystaUine forms are true gabbros 

 (" olivine-gabbros " of some authors) which graduate into dolerites, 

 and these in turn pass insensibly into basalt, magma-basalt, and 

 finally into basalt-glass. All these rocks have the same ultimate 

 chemical composition, and, so far as they are crystallized, are made 

 up of the same minerals. 



By the study of the microscopic structures of these rocks, it is 

 shown that the passage from the granitic type (gabbro) towards the 

 vitreous type (tachylyte) takes place along two distinct but parallel 

 lines. Sometimes the gabbro passes into opliitic dolerite, and this 

 into basalt and magma-basalt, with skeleton-crystals in the ground- 

 mass. At other times the gabbro passes into granulitic dolerite, 

 and thence into basalt and magma-basalts, with granular microliths 

 in their base. The first kind of transition is shown to be 

 characteristic of rock-masses which have cooled down with the 

 most perfect internal equilibrium ; the second kind is found in 

 masses which have been subject to internal movements during their 

 consolidation. All the rocks exhibit many interesting varieties of 

 the porphyritic structure, one of which, the " Glomero-porphyritic " 

 structure, has not hitherto been described. 



* Quart. Journ. Geol. See. vol. xli. (1885) p. 133. 



