academy of scE.vcEs] DISCUSSION AND CONCLUSIONS. 71 



juvenile solutions rich in soda, as is held by Flett and Dewey ('11) to have occurred in the same 

 manner but on a much greater scale than the albitization studied by Bailey and Grabham ('09) ; 

 it is, on the contrary, the result of the action of resurgent water. The rising magmas must pass 

 through wet sediments, and the water contained therein enters the magma, and assists in the 

 transfer of soda from the underlying normal basaltic magma to the upper portions which accord- 

 ingly become albitized. " Water must play an important role in modifying the magma in the 

 vents, and it seems impossible to doubt that occasionally the upper part of the magma-column, 

 and also some of the extrusive lava will become albitized." "As usual, special emphasis is laid 

 on the testimony of the sill," and Daly cites certain instances where the uppermost portions of 

 dolerite-sills are enriched in soda, though the bulk of the rocks are of normal composition. It 

 is not, however, shown that the instances quoted were formed under conditions comparable 

 with those specified by Flett and Dewey; and the "testimony of the sill," when applied by the 

 writer to a region in New South Wales, where the tectonic conditions of extrusion were exactly 

 as specified for the development of spilitic rocks, fails to support Daly's contention. (See p. 38.) 

 A great sill of dolerite about 1,500 feet thick is uniformly albitic from top to bottom. Moreover, 

 the spilitic suite is not throughout of basaltic habit, as Daly infers ('14, p. 339), but the spilites 

 are associated with other sodic rocks such as keratophyre, and hi New South Wales as in Britain 

 these form large independent masses which are composed predominately of albite, or acid 

 oligoclase (with a little quartz, augite, and magnetite) and are comparable in size with the 

 dolerite-intrusions themselves in some localities (Benson '18). There is clear evidence of strong 

 pneumatolytic action in connection with these rocks, which has converted large amounts of the 

 associated argillites into ferruginous jasper, and has affected the keratophyres themselves, but 

 this injection occurred after the argillites had become compacted (with elimination of much of the 

 contained water) and is usually found along lines of local fracture and small displacement 

 (Benson '15, '15a; cf. the Victorian "spilitic" jaspers, p. 37). The unfavorable conditions for 

 concentration of soda by resurgent water, the absence of evidence of such concentration in thick 

 dolerite-sills, and the occurrence of large independent intrusions of highly sodic keratophyre 

 supports the view that the immediate parent-magma, from which these New South Wales rocks 

 were derived was an unusually sodic one. The writer ('18) is, indeed, of the opinion that some 

 of the albite in the dolerites may be of primary origin, though dolerites of normal composition 

 are occasionally present, but recognizes the abundant evidence of the post-volcanic activity 

 of solutions in connection with the associated keratophyres. The New South Wales region may 

 therefore be held to exemplify the conclusions of Flett and Dewey. 



In those regions where the alkaline character of the rocks as a group is less clearly marked 

 or a mixed series of types is present, it seems possible that the "typical alkaline rocks" may be 

 extrusions from local magmas derived by a lengthy differentiation from the original magma (see 

 Harker's ('17) adaptation of Bowen's researches), and are intercalated with those of less 

 specialized magma-basins. Possibly as conditions of slow geosynclinal subsidence changed 

 toward those of orogeny, new drafts of the primitive would be raised into activity near the 

 surface, as a result of increased lateral pressure at depth, with the consequent development of 

 a mixed or transitional assemblage of rocks, as in the case of the Upper Devonian and Lower 

 Carboniferous igneous rocks in the Ural Mountains and in New South Wales. Bowen's work 

 would point also to the probability that conditions favoring the retention or escape of the 

 volatile constituents from the magma would exercise considerable effect in determining the 

 apparent " alkalinity " of the rocks developed. Perhaps in some obscure difference in the course 

 of regional differentiation of the subcrustal magmas in the Hercynian areas of southwestern 

 Britain and Germany may be found the explanation of the somewhat less markedly alkaline 

 character of the Devonian rocks of the latter region. Both series were erupted as extrusions 

 and intrusions under a thin cover of sediments that were then being deposited in subsiding 

 offshore regions. In both regions, also, this crust-sagging was the prelude to a great period of 

 lateral compression and orogeny. Hence the igneous rocks and sediments are highly dislocated. 

 In this they differ from the characteristic essexitic-theralitic rocks, with their picrites, which 



