[37] The Tertiary gabbros of the Inner Hebrides have already been cited, and will be more fully described in a later chapter as exhibiting the heterogeneousness of an eruptive magma.
[38] See Messrs. Dakyns and Teall, Quart. Journ. Geol. Soc. xlviii. (1892), p. 104; Prof. Brögger, op. cit. 1. (1894), p. 15; Mr. A. Harker, op. cit. p. 320; Prof. Iddings, Journ. Geol. Chicago, i. (1893), p. 833; Bull. Phil. Soc. Washington, ii. (1890), p. 191; 1892, p. 89.
As illustrations of these features I may cite two good examples, one from Scotland and one from England. The mass of Garabol Hill, in the Loch Lomond district, consists mainly of granite, occupying an area of about 12½ square miles. Messrs. Dakyns and Teall have shown that while the central portions consist of granite, the south-eastern margin affords a remarkable series of intermediate rocks, such as hornblende-biotite-granite, tonalite (quartz-mica-diorite), diorite and augite-diorite, which lead us outwards into highly basic compounds, including wehrlites (olivine-diallage rocks), picrites (olivine-augite rocks), serpentine (possibly representing dunites, saxonites, and lherzolites), and a peculiar rock consisting essentially of enstatite, diallage, brown hornblende and biotite. The authors regard the whole of these widely different rocks as the products of one original magma, the more basic marginal area having consolidated first as peridotites, followed by diorites, tonalites and granites in the order of increasing acidity. The most acid rock in the whole series consists of felspar and quartz, is almost devoid of ferro-magnesian minerals, and occurs in narrow veins in the granite and tonalite. It indicates that after the segregation and consolidation of the whole boss, ruptures occurred which were filled in by the ascent of the very latest and most acid remaining portion of still fluid magma.[39]
[39] Messrs. Dakyns and Teall, Quart. Journ. Geol. Soc. xlviii. (1892), p. 104.
The case of Carrock Fell in Cumberland has been described by Mr. A. Harker, who has ascertained that the gabbro of this boss has in its central portions a specific gravity of less than 2·85 and a silica-percentage sometimes as high as 59·46, whilst its marginal zone gives a specific gravity above 2·95 and a silica-percentage as low as 32·50. The migration of the heavy iron ores towards the margin is readily apparent to the naked eye, and is well established by chemical analysis, the oxides of iron amounting in the centre to 6·24 (Fe2O3 3·60, FeO 2·64), and at the margin to 25·54 (Fe2O3 8·44, FeO 17·10).[40] Neither in this instance nor in that of Garabol Hill has any evidence been noticed which would suggest that the basic and acid rocks belong to different periods of intrusion. They pass so insensibly into each other as to form in each case one graduated mass.
[40] Mr. A. Harker, op. cit. p. 320.
From these and other examples which have been observed, it is difficult to escape the conclusion that the differences between the basic margin and the acid centre are due to some process of segregation or differentiation while the mass was still in a liquid condition, and its constituents could pass from one part of the boss to another. According to Professor Brögger, it may be stated as a general law that differentiation sets in during consolidation, and is determined by, and dependent on, the laws of crystallization in a magma, in so far as the compounds which, on given conditions, would first crystallize out, diffuse themselves towards the cooling margin so as to produce in the contact-stratum a peculiar chemical composition in the still liquid material before crystallization takes place.[41]
[41] This general conclusion is stated by Professor Brögger from his investigation of the rocks of Gran, Quart. Journ. Geol. Soc. l. (1894), p. 36.
If during the process of differentiation, and before consolidation, injections of the magma occur, they may be expected to differ in character according to the portion of the magma from which they are derived. Professor Brögger believes that among the basic eruptive rocks of Gran in the Christiania district, one and the same magma has in the bosses solidified as olivine-gabbro-diabases, and in the dykes as camptonites, bostonites, pyroxenites, hornblendites, and more acid augite-diorites.[42]
[42] Quart. Journ. Geol. Soc. l. (1894), p. 35.