CHAP. VI 
VOLCANIC BOSSES 
91 
Loch Lomond district, consists mainly of granite, occupying an area of about 
12^ square miles. IMessrs. Dakyns and Teall have shown that while the 
central portions consist of granite, the south-eastern margin affords a remark- 
able series of intermediate rocks, such as hornhlende-biotite-grauite, tonalite 
(quartz-mica-diorite), diorite and augite-diorite, which lead us outwards into 
highly basic compounds, including wehrlites (olivinc-diallage rocks), picrites 
(olivine-augite rocks), serpentine (possibly representing dunites, saxonites, 
and Iherzolites), 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-maguesian 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.^ 
The case of Carrock Fell in Cumberland has been described by Mr. A. 
Marker, who has ascertained that the gabbro of this boss has in its central 
portions a specific gravity of less than 2 "8 5 and a silica-percentage some- 
times 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 (>-24 (Fe„Og3-60, FeO 2-G4), and at the margin to 25-54 
(Fe.,Og8-44, FeOI 7-10).^ Xeither 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. 
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 Brogger, 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 pro- 
duce in the contact-stratum a peculiar chemical composition in the still 
liquid material before crystallization takes place.^ 
If during the process of differentiation, and before consolidation, injections 
of the magma occur, they may be expected to differ in character according 
^ Messrs. Dakyns and Teall, Quart, Journ, Gcol. Soc. xlviii. (1892), p. 104. 
2 Mr. A. Ilarker, op. cit. p. 320. 
^ This general conclusion is stated by Professor Brogger from his investigation of the rocks of 
Gran, Quart, Journ. (Hoi. Soc. 1. (1894), p. 36. 
