10 THE MARQUETTE IKON-BEARING DISTRICT. 



as the Huron Islands, while inland they stretch westward as far as the 

 source of the Wisconsin River (p. 166). The larger portion of the Primary 

 series consists of granite or syenite (hornblende-granite). In its southeast- 

 ern portions granite is the predominant rock. Toward the northwest the 

 character of the series clianges almost imj^erceptibly. Quartz becomes less 

 and less abundant in the granite, and hornblende more abundant, until 

 finally the rock passes into a granular compomid of feldspar and horn- 

 blende, or into a greenstone. The granitic members of the series (which 

 are those included within tlie present ]\Iarquette district) are traversed in 

 all directions by greenstone dikes of various magnitudes that have produced 

 contact effects in the granite on both sides of them. These dikes are con- 

 nected with the great greenstone masses to the northwest. They are not 

 only of the same composition as the greenstone, but as the large areas of 

 the greenstone are approached the dikes become more and more abundant 

 m the granite, "until at length it becomes difficult to determine which of 

 the rocks predominates in quantity." Occasionally veins of other rocks than 

 greenstone are to be found cutting the granite, and "in a single instance 

 what was regarded as a true vein of poi'jjhyry, ha\dng a width of nearly 

 3 feet, was noticed, which Acin is crossed at angles of 53° and 107° by a 

 vein of greenstone, having a. width somewhat less than that of the j^orphyry. 

 In this instance the greenstone is clearly the most recent vein" (p. 176). 



It is evident that Houghton did not discriminate between the A-ounger 

 greenstone dikes in the granite and the greenstone-schist comprising the 

 large areas of greenstone to which he makes reference. The latter do 

 not send apophyses into the granite, but, on the contrary, the granite 

 sends dikes into the greenstone-schists. There is a gradation such as 

 Houghton describes, but the granite, and not the greenstone-schist, is the 

 invading rock. The small dikes in the granite are as described in the report, 

 but they are not genetically connected with the greenstone-schists. 



"Flanking the primary rocks on the south," writes Houghton, "is a 

 series of stratified rocks consisting of talcose, mica, and clay slates, slatv 

 hornblende rock, and quartz rock, the latter rock constituting by far the 

 largest jn-oportion of the whole group." Passing from the granite southward 

 near the lake shore, the series consists of a serpentine rock into which 



