GABBROS ETC. IN SCOTLAND AND IRELAND. 63 



y. Minerals of which the E-ocks are composed. 



In a previous paper* I have pointed out that the ultra-basic 

 rocks (dunites, picrites, eucrites, troctolites, Iherzolites, &c.), which 

 are so intimately associated with the gabbros and dolerites of the 

 Western Isles of Scotland, are made up of the same minerals as the 

 latter rocks, but grouped in different proportions. As I have already- 

 discussed the chemical composition and other peculiarities of these 

 minerals in detail, it will not be necessar}-' to do more than merely 

 refer to the results which were there arrived at. 



In making a study and comparison of crystalline rocks it is of the 

 utmost importance, as I have* already insisted, that we should dis- 

 criminate between the minerals which belonged to the rock at its 

 first formation and those which are of secondary origin. Many of 

 the erroneous and contradictory statements which have been made 

 by petrographers have resulted from a failure to recognize this fun- 

 damental distinction. At the same time, it must be admitted that 

 the distinction is not always an easy one to arrive at : indeed in 

 many cases doubts may remain after the most persevering study. 



Among the original minerals, it is clear, when we come to study 

 them with care, that certain crystals have separated out from the 

 magma at a period quite different from that of the formation of 

 other crystals. This fact has been very justly insisted upon by MM. 

 Fouque and Michel-Levy, who have pointed out the necessity for 

 discriminating between the several different periods of separation of 

 the crystals in a rock. The solution of this important problem must, 

 of course, be sought for in a study of the relations which the several 

 kinds of crystals are seen to bear to one another when the rocks are 

 examined in thin sections under the microscope. Further, if we 

 study the positions of the several crystals of the different minerals 

 which have separated during the same period of consolidatiou, we shall 

 find certain facts, such as the inclusion of the crystals of one mineral 

 in those of another species, or the assertion of the outward crystalline 

 form of one mineral at the expense of others, which enable us to 

 establish a distinct order of crystallization among them. 



The original minerals of the rocks may be conveniently divided 

 into two groups : those which are always present in every sample of 

 the particular rock, which we may call the essential constituents ; 

 and those which occur with greater or less frequency in addition to 

 the essential ones, which may be called accessory or accidental 

 constituents. 



The essential minerals of the most highly crystalline forms of these 

 rocks (gabbros) are three only, plagioclase felspar, augite, and olivine. 

 As the rocks, however, become less perfectly crystalline, magnetite 

 and a certain proportion of glassy ground-mass also take their place 

 as essential ingredients in them. 



The range in composition and characters of the essential minerals 

 of these rocks has been already pointed out. Chemical and optical 

 tests alike prove that the felspars vary in composition between 

 * Quart. Journ. Geol. Soc. vol. xH. (1885), p. 362. 



