STRUCTURAL GEOLOGY. 67 



Mnsoovadyte.] 



conditions, tend to separate themselves according to their specific gravity. Those 

 conditions must have been most favorable at that period when the cooling had not far 

 advanced, but had reached the stage at which labradorite was generated and remained 

 floating iii the still molten mass. The cooling proceeded very slowly and must have 

 taken place mainly after the severance of the Beaver Bay diabase. When that great 

 partition occurred it of course removed only the upper portion of the gabbro, and must 

 have carried along the major part of the feldspar masses. The inevitable result of such 

 a motion must have been the collision of the feldspar masses upon each other and 

 the rounding of their outlines. They seem to have been compressed and sometimes 

 broken. They were, perhaps, not rigid at first, but still flexible, allowing the 

 re-adjustment of the crystals, but also must have suffered numerous fractures and 

 warpings. Through the fractures the molten diabase entered, forming diabase dikes, 

 some of which still subsist, giving the impression of a later diabase intrusion. In 

 this slow movement of the great molten sheet, the feldspar masses which it carried 

 would first encounter obstructions from the underlying older rock. Becoming lodged, 

 temporarily or permanently, the still liquid magma would flow past them, thus 

 effectually removing them from contact with that particular part of the magma 

 which gave them birth, and increasing the petrographic contrast between them and 

 the diabase. 



Muscovadyte. If the gabbro mass be considered as an individual, having phases 

 of crystallization dependent on the stages of development, or as an epoch of geologic 

 history for that part of the state in which it is found, the muscovadyte and the 

 anorthosyte conditions occupy the extreme and opposite ends. One is so completely 

 differentiated that it can hardly be traced to its parent stock, and has been some 

 times denied all relationship in origin and in time with the source from which it 

 sprang. The other is so non-differentiated and complex that in like manner it has 

 been divorced from its offspring and has been assigned to an earlier geologic epoch 

 and another method of genesis. 



This rock presents protean characters, and its theoretical origin has had a 

 curious and checkered history since the members of the survey- began to study it. 

 When this rock was first encountered it was called " muscovado rock" without any 

 idea of its age and nature. Still, as it turns out, it had been examined earlier and 

 had received the name " impure quartzyte." Again it was described as " noryte," 

 and in one case it gave name to a point of land jutting into Clabimichigama lake, 

 although at that place it is largely a fine-grained pyroxenic and biotite gneiss. It 

 was found again to underlie, in form of angular fragments, a sheet of typical gabbro. 

 It was found to form a gradual transition into biotite mica schist. At this stage of 

 the investigation an effort was made to learn more definitely what distinctions were 



