252 Geological Excursion into Central Wisconsin — Hall. 



nesota as well as in Wisconsin these sandstones lie unconforraably 

 on crystalline schists, gneisses, granites and the Keweenawan 

 eruptives. 



Throughout this area of old crystalline rocks the descent of 

 the river is considerable. Point Bass, near which place the 

 gneisses disappear beneath the sandstones, is only a little over 900 

 feet above the sea. From its source to this place the T\'isconsin 

 river must fall about two feet to the mile for the distance it 

 flows, or about four and one-half feet to the mile for the actual air 

 line distance between the two points. This rapid rate of descent 

 is the occasion of great erosion and of numerous rapids and water- 

 falls along its course. These rapids afford disclosures of the crys- 

 talline rocks which were seen, and whose study furnishes most of 

 the material of this paper. 



It should be said in passing that this paper is not based on 

 that of Irving and Van Hise already referred to,* nor on the notes 

 which are published in another place,t and are supposed to embody 

 the results of the earlier investigations on these same and many 

 other rocks. The work here outlined is based wholly on the obser- 

 vations of the writer. The map, however, Plate iii, Fig. 1, is based 

 on the maps of the Wisconsin geologists, but with such modifica- 

 tions as the writer has thought best to add to that ground plan. 



The following rock types were noted: — 



Diabase. No exposure of this rock was seen in the whole 

 length of the valley traversed, an unexpected result considering 

 the fact that diabase dikes abound throughout the area of crystal- 

 line rocks in Michigan, Wisconsin and Minnesota. Some large 

 boulders occur on the island at Merrill just below Ihe dam which 

 are diabasic. They are very angular, which condition indicates 

 that they could not have been transported far. These boulders 

 exhibit a dark green or black rock of medium texture, weathering 

 to a gray color, a result due to a more rapid alteration of the py- 

 roxene and hornblende than of the plagioclase. Vein-like segre- 

 gations of pyrite appear on the fresh surfaces of the rock. In 

 addition to the weathering which the surface discloses, the interior 

 of this rock exhibits considerable change. The lath-like feldspars 

 have altered but little, save where cracks and opened cleavage 



*Crystalliiie Rocks of the Wisconsin Valley, by R. D. Irving and C. R. 

 Van Hise. — Geology of Wisconsin, Vol. iv, Part vii, pp. 625-714. 

 tibid, Vol. II, pp. 637-642. 



