GABBROS ETC. TN SCOTLAND AND IRELAND. t O 



structure * ; and the same structure is frequently exhibited in the 

 felspars of the gabbros of ^lull (see Plate VII. fig. 8). There are 

 strong grounds for believing, as Professor R. D. Irving remarked 

 in 1883, that some structures of this kind are of secondary origin in 

 the rocks t ; others, it can scarcely be doubted, are original. In rocks 

 from Portsoy I have found clear evidence of the origin of structures 

 of this kind, through alteration of the felspar (see Plate VII. fig. 9). 

 As these structures are much better exhibited in the more acid 

 rocks from the same district, I think it will be better to reserve the 

 fuU discussion of them until I come to the description of the latter. 



VIII. Geological Relations of the Rocks exhibiting the 

 SEVERAL Structures. 



AVhen we come to study in the field the positions and relations of 

 the rock-masses exhibiting the several structures above described, 

 some very important conclusions are suggested to us. 



The granitic structure is confined to the gabbros, and occurs only 

 in the central portions of the largest of the extruded masses. The 

 very coarse-grained and porphyritic forms occur in the central parts 

 of the great mountain-masses of Skye and Ardnamurchan, or occa- 

 sionally as contemporaneous veins traversing finer-grained rocks of 

 the same class in these and the other districts. 



From the granitic types, through less perfectly crystalline forms, 

 to the vitreous there is, as I have always insisted, the most perfect 

 gradation. But the careful study of a large number of microscopic 

 sections, cut from rocks the positions and relations of which have 

 been carefuUy determined in the field, brings out a remarkably 

 interesting fact. The passage from gabbro to glass takes place not 

 along one line simply, but along two distinct and parallel ones. 



Sometimes we find about the edges of the great mountain-masses 

 the felspars losing their broad platy character and exhibiting lath- 

 shaped sections. In these cases the felspars are found becoming 

 more and more completely enclosed in the pyroxenes, until the most 

 perfect ophitic structure is produced. It is possible in this way to 

 illustrate in the completest manner the transition from the most 

 perfect gabbro to the most typical ophitic dolerite (see Plate IV. 

 figs. 3, 5, & 7, and Plate V. fig. 1). 



But at other times the change from gabbro to dolerite takes place 

 in a totally different way. The felspars, as in the last case, exchange 

 their broad platy forms for narrow prismatic ones with lath-shaped 

 sections ; but these, crowding together into an intercrystallized 

 mass, have had developed in their midst a number of grains of 

 pyroxene and olivine, everywhere distinguished, as seen in section, 

 by their curvilinear outlines. In this way we have a granulitic 

 dolerite formed (see Plate IV. fig. 8, and Plate V. fig. 2). 



Let us now inquire if there is anything in the geological position 



* Loc. cit. p. 33, pi. vi. fig. 2. 



t U. S. Geological Survey, Monograph V. The Copper-bearing Eocks of 

 Lake Superior (1883), p. 114, pis. xiv., xy. 



