92 Reports and Proceedings — 



In previous papers published in 1874 and 1876, it has been demon- 

 strated by the author that there exist in Scotland and in Hungary 

 igneous rock-masses presenting the most perfectly crystalline 

 characters and belonging to the Tertiary period. It was further 

 shown that such highly crystalline, plutonic rocks are seen passing 

 insensibly into volcanic rocks of the same chemical composition — 

 gabbros into basalts, diorites and quartz-diorites into andesites, and 

 quartz-andesites and granites into rhyolites — the lavas in turn 

 graduating into the perfectly vitreous types known as tachylytes and 

 obsidians. 



The present paper deals with the basic rocks of Western Scotland 

 and Northern Ireland, which are shown to exhibit the most marked 

 analogies with rocks of the same age in the Faroe Isles and Iceland ; 

 these facts lend strong support to the doctrine of the existence of 

 petrographical provinces. The Tertiary age of the Scotch and Irish 

 rocks is placed beyond dispute by the fact that they overlie uncon- 

 formably the youngest members of the Cretaceous system, and are 

 interbedded with stratified deposits of Lower Tertiary age. 



With regard to the nomenclature of these rocks, the identification 

 of the more crystalline forms with the gabbros, which was made by 

 Zirkel and Von Lasaulx, is supported ; while the use of the term 

 " dolerite " as a convenient one for the connecting links between the 

 gabbros and basalts is advocated. 



Of the original minerals contained in these rocks, plagioclase 

 felspar (ranging in composition from anorthiteto labradorite), augite, 

 olivine, and magnetite, are regarded as the essential ones ; while 

 enstatite, biotite, chromite, picotite, and titanoferrite are among the 

 most frequently occurring accessories. It is shown, however, that 

 these original minerals may belong to different periods of consolida- 

 tion. The Secondary minerals are very numerous, including quartz, 

 epidote, zoisite, hornblende, serpentine, and zeolites, with many other 

 crystallized and uncrystallized substances. There are remarkable 

 variations in the relative proportions of the original minerals in 

 different examples of the rock ; and by the complete disappearance 

 of one or other of the constituents, the gabbros are sometimes found 

 passing into picrites, eucrites, or troctolites. 



In their microscopic structure these rocks present many interesting 

 features. From the highly crystalline gabbros there are two lines 

 of descent to the vitreous tachylytes : one through the opliitie 

 dolerites and basalts, and the magma-basalts with skeleton-crystals ; 

 and the other through the granulitic dolerites and basalts, and the 

 magma-basalts with granular microliths. The former are shown to 

 result from the cooling down of molten masses which were in a state 

 of perfect internal equilibrium ; while the latter were formed when 

 the mass was subject to movement and internal strain. 



It is shown that in the most deeply seated of these rocks (gabbros) 

 the whole of the iron-oxides combine with silica; but, as we approach 

 the surface, the quantity of these oxides separating as magnetite 

 increases, until it attains its maximum in the tachylytes. In all the 

 varieties the order of separation of the different minerals is shown 



