On the Gabbros, Dolerites, and Basalts of Tertiary Age, 161 



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 anorthite to 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 consolidation. 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 com- 

 plete 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 inter- 

 esting features. Erom the highly crystalline gabbros there are 

 two lines of descent to the vitreous tachylytes : one through the 

 ophitic 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 (gab- 

 bros) 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 not to depend solely on chemical causes, but to be in- 

 fluenced by the conditions under which the rocks have cooled down. 



Although these rocks are not highly altered ones, yet they afford 

 admirable opportunities of studying the incipient changes in their 

 constituent minerals. The nature of these changes is discussed, 

 and they are referred to the following causes : — (1) the corrosive 

 action of the surrounding magma on the crystals ; (2) the changes 

 produced by solvents acting under pressure in the deep-seated 

 masses (these have been already described under the name of 

 " schillerization ") ; (3) the action of heated water and gas escaping 



Phil. Mag. S. 5. Yol. 21. No. 129. Feb. 1886. M 



