academy of 8CIEN08 ] DISCUSSION AND CONCLUSIONS. 73 



THE LACCOMORPHIC COMPLEXES. 



These are most fully illustrated by the discussions of the Duluth, Sudbury, and Bushveld 

 complexes. We may consider several hypotheses explanatory of the mode of differentiation 

 of these. Coleman ('07) , Daly (' 14) , and Bowen ('15) consider these -are resulting from gravita- 

 tive differentiation in situ, "with some movements during consolidation resulting in a certain 

 amount of injection of one type into another. * * * In some localities more than one prin- 

 cipal intrusion of basaltic magma took place, and therefore two or more interlocking gabbro- 

 granite sequences may exist with resultant irregularities in the order of succession" (Bowen 

 '15, p. 55). Grout ('18a) urges that there is not a regular sequence of differentiated types in 

 these complexes, but that the acid upper portion is sharply distinct from the lower basic zone, 

 which latter is often strongly banded. He suggests that the granite may have been separated 

 gravitationally from the gabbro and become immiscible with it, due perhaps to the concen- 

 tration of water within it, while the gabbro becomes banded, as a result of convection-currents. 

 Bowen's ('19) alternative explanation has been given above (p. 54), and is based on the 

 relative movements of solid and liquid in the crystallization of single originally uniform gab- 

 broid magma, which by warping permits the accumulation of liquid residue in the planes 

 of tension in a crystal-mesh, leading to the formation of a banded structure parallel to 

 the floor of the intrusive mass, while a moderate amount of lateral compression when the rock 

 is almost completely crystallized determines the separation of the upper layer of granitic 

 material by the squeezing upward of the last residue of interstitial liquid from the crystal- 

 mesh. This liquid can not apparently be more than 35 per cent of the whole magma, and is 

 probably much less. "It is suggested that the various kinds of filter-press action (discussed) 

 probably can not take place before the mass is 80 per cent crystalline." 



We may now apply this conception to the case of the Bushveld complex. First it must 

 be noted that though the thickness of the granite mass, and therefore its total bulk, can not be 

 determined, its areal extent in proportion to that of the basic rocks is very much greater than 

 in the case of the relation of the areas of acid and basic rocks in the Duluth region, though the 

 compositions of the rocks are analogous. We must, however, note Bowen's remark ('19, p. 

 408) that there is no necessity for the constant relative proportion of their amounts. The 

 basic lower portion of the complex is strongly banded, and yet over wide areas these bands, 

 which are parallel to the floor of the complex, are so nearly horizontal that it seems rather 

 heroic to attempt to explain their origin as the result of repeated warpings of the floor during 

 the crystallization of the magma. Much more inadequate seems the hypothesis of convection- 

 currents. Again after the acid portion had become completely solid, was exposed by erosion, 

 and covered again by sediment, plutonic activity was resumed, and granite of the same nature 

 as previously, rose through the earlier granite and invaded the new formed sediments. It is 

 difficult to conceive it possible that this came from within the Bushveld complex; it must be 

 considered to have come from below, where the old parent-magma remained at least poten- 

 tially molten. We have already pointed out such long suspensions of activity in a single cycle 

 of vulcanicity in Scotland (p. 12) and New South Wales (p. 38) and the long duration of the 

 potential activity of a deep-seated magma is thus apparent. It may, therefore, be permissible 

 to inquire whether the first granite could not also have come from these deeper levels after 

 the basic rocks, and "slid in on top of its forerunner." Thus Brouwer ('17) thinks "we could 

 explain the facts in a rather satisfactory way if we admitted effusion (of acid volcanic rocks) 

 and several intrusions from a deeper-seated mother-magma." The question is not one that 

 can be answered here from a review of the literature only, but it may be noted that the fact 

 that the chilled margins at the base of the laccolite and the smaller associated sills are doleritic, 

 accords with the view that the first magma to enter the laccolite chamber was normal basaltic 

 magma" and that some differentiation occurred in situ. (See postscript, p. 78.) 



