72 THE GEOLOGY OF MINNESOTA. 



[The origin of the gabbro. 



3. The lines of granitic protrusion and intrusion are found to be limited, in 

 their main dimensions and directions, by the known earlier existence of areas of acid 

 fragmentals, only encroaching in the form of apophyses, and rarely, as rocks of 

 intermediate acidity, on the areas of earlier basic rock. Hence, again, if the com- 

 parison hold good, the gabbro should be found in a belt of Archean basic rock, or in 

 the extension of that belt. 



The first two of these statements are so evident from a moment's examination 







that they need no amplification. It may be well, however, to call attention to the 

 probable further westward extension of the gabbro than is usually represented, and 

 in a direction more westerly. Its surface exposure is cut off by a heavy drift 

 covering. It is likely also that the spur of the gabbro belt which appears at Duluth in 

 large part belongs to the Beaver Bay diabase which left the main mass by a grand 

 movement toward the basin now occupied by lake Superior, and hence that the main 

 trend of the gabbro body runs to the north of Duluth and toward the mouth of the 

 Cloquet river in the St. Louis valley. Toward the northeast the gabbro fades out by 

 running into a series of sills and dikes in the Animikie, and in this form it appears 

 for many miles in Canada, there being no general gabbro area free from the Animikie. 

 In respect to the third of the foregoing general comparisons, it should be noted, 

 in the first instance, that the uniform directions of the metamorphic belts of extreme 

 metamorphism, which are approximately outlined by the ranges of granite, show a 

 tendency to change as they approach the basin of Lake Superior. This is more 

 apparent after the epoch of the gabbro than before it. Whatever may have been 

 the cause or causes of this shifting in the direction of the later zones of metamor- 

 phism and eruption, it is apparent that it resulted in the more definite outlining of 

 that basin in the later stages of its geologic history. It is apparently in keeping 

 with this gradual change in the lines of metamorphic intensity, that the gabbro has 

 its more southerly position and its somewhat crescentic shape, with its concavity 

 toward lake Superior. The question now arises whether, where the gabbro now 

 occurs a belt of Archean greenstone formerly existed. The line of direction of the 

 northerly boundary of the gabbro between Gunflint lake and Kawishiwi river 

 (T. 63-9 W.) is in contact with greenstone continually, and it is in this interval that 

 are found its muscovadyte phases. Further southwest, and to Birch lake, it lies 

 alongside the eastern part of the White Iron Lake granite. But it is known that this 

 granite was intrusive at an earlier date in greenstones at White Iron lake and 

 southwestwardly at Birch lake. It is hence to be inferred that the general greenstone 

 belt extended originally at least to Birch lake. At the south of Birch lake the 

 Animikie reappears. It lies on the Keewatin along. the southern slope of the Giant's 

 range; and this Keewatin, largely of^fgreenstone character, is known to continue 



