258 Geological Excursion into Central Wisco7isin — Hall. 



Hypersthene appears here in about the same proportion as 

 there. It alters here, as there, by the corrosion of its borders 

 and the change of its material into a liornblendic substance, and 

 by the alterations along fractures and cleavage planes, into a 

 fibrous green product, with the fibres standing at right angles to 

 the corroded surfaces. The pleochroism of the hypersthene in the 

 sections from this locality is unusually weak. 



Olivine was noted in one or two slides. 



Quartz Diorite. The rocks at Merrill and Mosiuee which are 

 here grouped as gabbroids are really, in their present condition, 

 simply quartz diorites. They possess a generally massive structure 

 and medium texture, but they vary somewhat in both these char- 

 acters. For instance, at Merrill, on the island, where this rock 

 comes in contact with the gneiss, it exhibits a distinctly laminated 

 structure. Where this structure appears much jointing is also 

 present, and the major system is parallel with the plane of contact. 

 In other places the lamination is more obscure. Everywhere the 

 rock is very tough and firm, and usually it is more than one-half 

 hornblende. At Mosinee, on both sides of the river at Little Bull 

 falls, and on the island, the dark colored massive rock prevails. 

 Normally it contains about the same proportion of hornblende as 

 does that at Merrill. Here and there is more feldspar or a coarser 

 texture than the average. No lamination was seen at this point, 

 save in the neigborhood of a large quartz vein. This vein, which 

 carries white quartz, has been worked somewhat for gold. 

 It is in places much split up and faulted; one fault throws the 

 vein 30 or 40 feet from its course. The rock here is thoroughly 

 shattered. The ''nests of a very fine-grained and compact black 

 rock," which Irving mentions* are seen on the east side of the 

 river near the railroad tracks. 



It is unnecssary to repeat here the microscopic descriptions of 

 these rocks which the Wisconsin geologists have given.f The 

 conclusion drawn from an examination in the field and with 

 the aid of thin sections, is that they are altered gabbros. The 

 present constituents, save possibly the older generation of feld- 

 spars and the pyroxenic cores still remaining, are wholly sec- 

 ondary, due in part to the infiltration of chemical matters, but in 

 very large part to the molecular transformations, and conse- 



*Ibid, Vol. IV, p. 651. 



tibid, Vol IV, pp. 655-657; 706, 707. 



