74 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON fM£M0,E ^ A ™xix; 



THE CORDILLERAN COMPLEXES. 



Laccomorphic complexes occur where there is no dominating lateral pressure, and gravity- 

 control of the differentiation may appear more or less clearly, but as lateral pressure increases 

 differentiation by deformation becomes the controlling factor in the way Dr. Harker and Dr. 

 Bowen have described. Where the pressures were irregular "a wholly random spatial arrange- 

 ment" of the several differentiates may be brought about, but where they are more uniform in 

 direction the various rock-types in the complexes tend to become arranged in a definite manner, 

 and largely to consist of separate intrusive masses. The basic and especially the ultrabasic 

 rocks tend to form concordant sheet-like masses, the more acid, possibly as a result of the power 

 of stoping possessed by their lighter magmas, making bosses with transgressive boundaries; but 

 in cases of very great lateral pressure even these from more or less concordant intrusive masses. 

 The possibility of differentiation of these in situ, of the limiting of the effects of such differentia- 

 tion according to the size of the itrusive mass, and the formation of chilled basic marginal 

 phases, have been discussed by Daly, Bowen, and others, and need not be considered here. 

 Both with these and the laccomorphic complexes dikes of ultrabasic rocks may occur among 

 the latest members of the retinue of subsequent minor intrusions. 



THE ERUPTTBHITY OF PERIDOTITE. 



Bowen ('15, p. 31) has indicated that in a magma from which olivine crystals were settling, 

 the total composition of the basal portion might become lherzolitic while it still contained 

 nearly 50 per cent of liquid, so that it would still be eruptible, but a pure olivine-magma (or a 

 pure anorthosite-magma) could not be developed by differentiation from a normal basaltic 

 magma (Bowen, '17, '19). He further states that the facts of serpentinization adduced by 

 the present writer do not indicate the presence of such an excessive amount of water in the 

 ultrabasic magmas as could warrant the assumption that a magma, which could crystalize com- 

 pletely as dunite, could be so fluxed, though the presence of some water does seem to be 

 indicated, notably in the case of the peculiar veinlets of serpentine in the stubachite of Wein- 

 schenk (Benson, '18, p. 718). Hence Bowen ('20) considers that a sill or dike-like mass of dunite 

 might result from crystallization from normal basaltic magma, where by subsequent movement 

 the residual magma has been squeezed out, connoting the probable association of such ultra- 

 basic masses with granitic differentiates in the widened portions of the same intrusive body. 

 The question is best investigated in the Tertiary complexes, for in the older and more deeply 

 eroded masses of igneous rocks the lighter differentates might have been completely removed. 

 In Rum, minutely described by Dr. Harker ('OS p. 68) no evidence of fluxing by magmatic 

 water can be recognized, but the gneissic structures present do seem to accord with those which 

 might be expected in rocks crystalllized from magmas containing a large proportion of crystals 

 at the time of their injection, though the presence of later and more acid differentiates of the 

 primitive magma successively below this, seems to show clearly that the peridotite was not 

 formed by differentiation in situ during deformation, but resulted from the completion of 

 crystallization in a periodotitic magma injected as such into its present position, the first of the 

 fractional magmas drawn off from the immediate parent reservoir below. Again in the case of 

 New Caledonia, where there is an area of 2,600 square miles of peridotite, more or less ser- 

 pentinized, and intrusive into Eocene rocks, Piroutet has shown that it forms massives of varying 

 size, which send out numberless sills and apophyses. Card's ('00) description of a few speci- 

 mens from this locality shows that they are composed of olivine and enstatite, though some 

 dunite is present. The writer also has noticed that of a collection of serpentines from here the 

 greater part were derived from harzbergite or lherzolite. Gabbro occurs in a few masses only, 

 and rare scattered pebbles, derived doubtless from small inconspicuous intrusions, form almost 

 all that is known of the more acid rocks. There can be no doubt that here great masses of 

 peridotitic magma rose as such into the upper portions of the earth's crust, and its differentiation 

 in situ was confined to the separation of the different types of peridotite and dunite from one 

 another. It was not a mono-mineralic intrusion, but one doubtless explicable on the first of 



