INDUSTRIAL PROGRESS DURING THE YEAR 1876. xcv 



Blake in the West {Record^ 1874, Ixxi.) have made known the exist- 

 ence of repeated intercahitions, by successive outflows, of trachytic 

 and doleritic rocks. Marvine has since pointed out another instance 

 of the kind in Nevada, where two or more beds of dark-colored 

 olivenitic basalt, with interposed sandstones, are capped by a great 

 mass of pinkish trachyte or rhyolite holding sanidinc and quartz, 

 and including pebbles of the underlying basalt. 



CRYSTALLINE ROCKS OF MICHIGAN. 



The crystalline schists of the northern iDcninsula of Michigan have 

 been again discussed by Brooks, who, after the Canada Survey, has 

 referred them to the Huronian period. He calls further attention to 

 the existence at the summit of the series of a group of granitic rocks, 

 with hornblendic and micaceous schists holding staurolite and gar- 

 net, the whole being, according to him, aj^parently younger and dif- 

 ferent in character from any of the known Huronian rocks. The 

 writer had already described these rocks in 1871, and then referred 

 them to the White Mountain or Montalban series, of wdiich they have 

 both the lithological characters and the geognostical relations. 



THE KEWEENIAN SERIES. 



The series of sandstones and trappean rocks which carry the 

 native copper of Lake Superior has been by Irving studied in Wis- 

 consin, where is found a westward extension of the Lake Superior 

 synclinal of these rocks, and where they have an apparent thickness 

 of more than 20,000 feet. They are there as clearly distinct from the 

 underlying Huronian series as from the overlying horizontal sand- 

 stones of Lake Superior, which are, according to Irving, traced west- 

 ward beneath the Lower Paleozoic sandstones of the Mississippi Val- 

 ley, of which they are apparently inferior members. Brooks now 

 concludes that the granitoid and micaceous formation just noticed, 

 which he places at the summit of the Huronian, is unconformably 

 overlaid by the copper-bearing formation, which he proposes to 

 recognize as a distinct series under the name of Keweenawian. The 

 wTiter had, however, already in 1873 named it the Keweenaw series ; 

 and if an adjective is desired, Keweenian will be more eujDhonious 

 than the word proposed by Brooks. The evidence, then, is to the 

 effect that the succession in this region is: 1. Laurentian; 2. Hu- 

 ronian ; 3. Montalban ; 4. Keweenian ; 5. Lower Cambrian to which 

 period the overlying sandstones must be referred. Tliis disposes of 

 Bradley's late strange suggestion that the Huronian is probably 

 altered Lower Silurian that is to say, Lower Cambrian. 



CRYSTALLINE ROCKS OF PENNSYLVANIA. 

 Hunt has described the crystalline rocks of the South ]\Iountain 

 between the Delaware and the Schuylkill, and of the Welsh Mount- 



