76 BASIC AND ULTKABASIC IGNEOUS KOCKS-BENSON. 1Memoir 1vo L t Tix. 



and the tectonic conditions tend toward the formation of a folded eordillera, the successive 

 intrusions are more or less separated from one another, and the more basic, earlier intrusions 

 are more closely concordant with the structure of the country than the later more acid series 

 of intrusions, the mise-en-place of which may have been more largely determined by stoping. 

 This is perhaps part of the explanation of the processes that make Suess's statement that 

 " the green-rocks form sills in dislocated rocks, which sometimes followed the bedding plane and 

 sometimes the plane of movement," so frequently an accurate expression of the facts. The later 

 acidic intrusions are batholiths, with strongly transgressive boundaries when considered in detail, 

 though their major axes, or distribution, generally will conform to the structure of the country. 

 The later Paleozoic plutonic rocks of northeastern New South Wales (fig. 10, p. 54) form a strik- 

 ing illustration of this. Moreover the sill-like ultrabasic rocks occur in the outer portion of the 

 folded range, while the granites invade the central portion, as Suess also notes in connection 

 with those of the younger mountain ranges (IV, p. 561). Similarly also we may note that 

 the ultrabasic rocks of the complex of the Harz Mountains (fig. 6), a region in which the folding 

 was directed toward the northwest, lie to the northwest of the granite massif, and extend in 

 a southwest to northeast direction for the most part, i. e., concordant with the axes of folding. 

 In the Taurus mountains also (fig. 8a), the thrusting of the ultrabasic and basic magmas to 

 the outer portion of the folded range is very marked. 



The hypothesis thus outlined seems also to be compatible with the occasional intrusion of 

 granitic masses beneath the basic rocks, as in the west of Scotland (figs. 3 and 4), and with the 

 possible development of hybrid rocks between the gabbros and granites in the Bushveld, 

 Duluth, and Sudbury complexes. It does not involve any extensive assimilation of invaded rock, 

 nor deny the possibility of a minor amount of such solution. It does not involve the assumption 

 of immiscibility of magmas, and thus accords with the evidence of experimental research. It 

 recognizes the occurrence of masses differentiated in situ. It is, perhaps, not incompatible 

 with the explanation suggested by Bowen for the Tagil complex (fig. 5), though here we must 

 notice that the proportion of acid to basic rock is immensely greater than that suggested in the 

 discussion of the Duluth mass, so far as the areas of exposure afford an indication of relative 

 volume, and that a reservation may be made concerning the acceptance of the pyroxenite as a 

 "huge reaction-rim" about the peridotite. It seems also to give the opportunity for horizontal 

 differentiation in response to lateral thrust, which, in a very broad sense, is required by the 

 Harker-Becke hypothesis of the development of characteristic petrographical provinces, a 

 differentiation, the mechanism of which has been greatly elucidated by the work of Smyth 

 ('13) and of Bowen ('15). This same mechanism, carried to an advanced stage, might produce 

 the association of alkaline with subalkaline rocks in regions, where, in the absence of domi- 

 nating lateral thrust, gravity exercised the greater influence on the course of differentiation in 

 the deep-seated magma-reservoirs, and may therefore explain the mixed character of the suc- 

 cession of rocks erupted during the sagging of the crust which precedes orogeny. 4 



SUBALKALINE AND ALKALINE PETROGRAPHICAL PROVINCES. 



This discussion brings us to the question of the reality or significance of the separation of 

 petrographical provinces into two types characterized by different tectonic associations. 

 It is unfortunate that some criticism of this hypothesis does not appear to consider the essential 

 feature in it. Thus there have been cited (Cross '10, '15) as examples of associations of rocks 

 believed to be adverse to the hypothesis, the Tertiary alkaline rocks of the Bohemian Mittelge- 

 birge, and the adjacent Paleozoic (or pre-Paleozoic) gneisses and calcic granites and gabbros, 



> Following the usual custom, we have in the foregoing referred to the "overthrusting" of folded strata. Hobbs ('14) has, however, argued 

 that such movement is essentially underthrusting in an opposite direction to the movement usually assumed. This is not opposed to the con- 

 ception of the thrusting out of residual magma in the direction of assumed overthrust. If we note the frequency with which dynamic metamor- 

 phism decreases in this direction, it seems probable that, whatever may be the distribution of forces at great depths, the lateral pressure at the 

 depths where the intrusive masses of magma differentiate is perhaps the upper member of the folding couple of forces, and does decrease in the 

 direction of assumed overthrust, with the resulting distribution of the fractions of the differentiated magma. Hobbs's suggestion, indeed, gives 

 the added possibility, which should be kept in mind, that magma situated at still greater depths may be differentiated by the "filter-pressing'! 

 out of the residual magma in a direction opposite to that of the assumed overthrust. 



