STRUCTURAL GEOLOGY. 71 



The oriRin of the Rabbro.l 



have led them face to face continually with the gabbro alliance of the same. Prof. 

 Bayley, in one of his earlier discussions, was so impressed with this alliance that he 

 grouped not only the original greenstone and the muscovadyte as parts of the gabbro 

 (much altered), but also insisted that the siliceous or quartzyte phase was only a 

 silicitied part of the base of the gabbro;* and in support of the latter he quoted the 

 observations of several officers of the United States geological survey. Several years 

 ago it was agreed by the members of the Minnesota survey, after a petrographic 

 examination of specimens from various localities, that the muscovadyte proper is a 

 phase of the gabbro, and that the name should be restricted to apply only to a rock 

 directly connected with and presumably derived from the gabbro. f 



The following is quoted from the report where this conclusion is published: 



" It would appear from the foregoing that the term muscovado rock (or muscovadyte) has been applied in 

 the field to rocks of different stratigraphic position and origin. This has already been asserted by Mr. H. V. 

 Winchell in the seventeenth annual report, pages 130, 131. It is also apparent that one of these is produced by 

 the action of the gabbro upon the sedimentaries. It appears also probable that the southern belt of muscovado 

 [ i. e. that at Muscovado lake] is a phase of the gabbro proper, and that, if to either the name should be continued , 

 it should be applied to this southern belt. 



" There remains, however, this possibility, if not probability, that this southern muscovado represents the 

 profound action of the true gabbro upon a basic Archean greenstone which has been brought to the surface in 

 the midst of the gabbro area by a later fault. We have learned from numerous observations that all the rocks 

 in this region have in some places been extensively faulted. It will be well, therefore, still, before adopting this 

 duplicate theory of the origin of the so-called muscovado, to examine further critical specimens collected at 

 points where it can be shown that the true gabbro was superposed upon a basic greenstone of Archean age." 



So far as possible since that date the term has been so used; but on making an 

 exhaustive microscopical examination of the specimens collected, and of the notes 

 and descriptions of all the members of the survey, both published and unpublished, 

 the conviction returns that the muscovadyte is also allied with the Archean. 



To account for the gabbro, therefore, is to find some way to explain the con- 

 version of an Archean greenstone with its siliceous accompaniments into that rock. 

 There is only one recourse metamorphism, carried to fusion and intrusive action. 

 This is the same principle appealed to and already adopted to account for the 

 Archean granites. 



Besides these petrographic and special considerations indicating such genetic 

 relation, there are some broader inferences to be drawn from the general geology of 

 the northern part of the state. 



1. The great area, the manner of occurrence of the gabbro body and its wholly 

 crystalline condition, indicate that in method of genesis it is comparable to that of 

 the igneous granites, and hence that it should be found in the zone of a dynamic 

 strain, or at least in line of a metamorphic belt. 



2. The earlier metamorphic belts in the northeastern part of the state trend 

 northeast and southwest, hence, if the comparison hold good, the direction of the 

 greater axis of the gabbro belt should extend northeast and southwest. 



* Ninetcentli Report, pp. 193-210. 



t Twenty-first Annual Report, pp. 143-152, 1892 [1893]. 



