Page:The Journal of geology (IA journalofgeology11893univers).pdf/110
erate is a quartzite which includes the Mt. Mesnard and Teal Lake quartzites. In Sec. 20, T. 47 N., R. 26 W., and Secs. 8 and 19, T. 49 N., R. 28 W., near Silver Lake and in other places, sediments of the Holyoke formation have sifted down into the fissures and joints of the preëxisting rocks, when they have a dyke-like character. For such formations the term "clasolite" is proposed. The dolomite of Mt. Mesnard and thence to Goose Lake, while lithologically, like that placed in the Republic formation, is doubtfully referred to the Holyoke. Argillite, graywacke and mica-schist occur extensively in the Holyoke, constituting the upper horizon. It is doubtful whether any granite or felsite of Holyoke age exists in the Marquette district.
Diabase, diorite, porodite, and peridotite occur abundantly, belonging both to the Republic and Holyoke formations. According to Mr. Seaman, diabase dykes of the Gogebic area are probably the same as those that cut the overlying sandstones of the Keweenawan, from which it is concluded that the Keweenawan lava flows are the effusive equivalents of the Holyoke diabase dykes.
The soft hematites of the region are produced by secondary enrichment at places where the water could best act, being at points of fracturing or in basins. The silica of the lean material has been leached out, and in its place iron oxide substituted. Gold and silver veins are discussed, and a classification of ore deposits given.
The Eastern or Potsdam sandstone rests unconformably on the Azoic. This includes the unaltered horizontal sandstone, which is free from dykes of eruptive material, and the Keweenawan, which consists of lava flows alternating with sandstones and conglomerates, largely derived from the former. Above, and conformably with the Eastern sandstone, near L'Anse, is limestone of Silurian age, as shown by its fossil contents. On Keweenaw Point the Eastern sandstone dips toward, and passes under, the interstratified sandstones and lavas of the Keweenawan. At or near the contact is a fault. However, at Douglas, Houghton and Hungarian rivers, it is thought not to be at the contact, and consequently that the Eastern sandstone underlies the Keweenawan lava, but the Eastern standstone may contain two or more sandstones of different ages, which may perhaps be considered as the most probable explanation of all the evidence. In Sec. 13, T. 16 N., R. 41 W., on the South Trap range, a nearly horizontal, soft, friable micaceous sandstone is found near the interbedded Keweenawan melaphyr and indurated sandstone. This soft sandstone contains numerous spherical spots very common in the Eastern sandstone, but not found in the Keweenawan. In the soft sandstone are found pebbles and large angular fragments of indurated sandstone, which Mr. Seaman thinks could only have been derived from the adjacent indurated sandstone. The rocks of the Trap range here exposed are believed by Mr. Seaman to hold a position near the top of the Keweenawan series, and he