GABBEOS ETC. II^" SCOTLAND AND IRELAND. 67 



In their mineralogical constitution these rocks also present 

 remarkable analogies ; and these come out more strikingly if we make 

 due allowance for the different kinds of alteration which minerals 

 undergo when they exist in deep-seated igneous masses and in 

 lavas poured out at the surface. All these rocks, except such as 

 are perfectly vitreous in structure, contain plagioclase felspar, 

 augite, and olivine. In the deeply-seated forms, the augite, as the 

 result of secondary changes, is often found converted into diallage, and 

 the olivine is more or less completely obscured by the develop- 

 ment of secondary magnetite. In rocks which have consolidated 

 nearer the surface, more or less magnetite has separated from the 

 original magma, and a greater or less proportion of the latter has 

 consolidated, without crystallization, into a vitreous ground-mass. 



It is in their structure that we find the chief distinctions between 

 the several groups of the basic rocks; and the main object of this 

 memoir is to inquire whether there is anything in these structures, 

 as studied by the aid of the microscope in the different varieties, 

 which is opposed to the conclusions arrived at by observations in 

 the field. 



These field-observations point most distinctly to the conclusion 

 that the degree of crystallization of an igneous rock affords no 

 criterion as to its geological age, but is determined by the distance 

 from the surface at which it has cooled down. In the district 

 specially described by me it was shown that the most highly 

 crystalline varieties of these rocks originally formed the centres of 

 the largest volcanic cones, such as those of Skye, Ardnamurchan, 

 Mull, and the Carlingford district. In the peripheral portions of 

 these great mountain-masses, as well as in the centres of similar but 

 smaller masses, and in sheets and large dykes proceeding from them, 

 we find a less perfect crystallization of the materials. In the lava- 

 streams and smaller dykes, clearly connected with the same centres 

 of eruption, we find more and more of the materials remaining in 

 the uncrystallized or vitreous condition ; while in situations where 

 very rapid cooling has evidently taken place, as on the sides of 

 dykes, basalt-glass has been formed. 



I shall now proceed to show, from the microscopic study of a 

 very large series of specimens, collected in all parts of the district, 

 that the study of these rocks in the laboratory fully bears out the 

 conclusions arrived at in the field. In classifying the several rock- 

 structures, I have been led to adopt a nomenclature almost identical 

 with that proposed by MM. Touque and Mich el- Levy, and now 

 very generally employed by French geologists. But I shall show 

 that, distinct as these types may appear to be, they really pass into 

 one another by the most insensible gradations. It wiU be my 

 farther aim to determine the causes by which the same magma 

 cooling down under diverse conditions, is made to assume such 

 remarkably different structures. 



The chief structures which I have been able to define in the 

 rocks under consideration are as follows : — 



1 . The Granitic Structwe (see Plate lY.). — In the rocks exhibiting 



p2 



