PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 789 



Clay stones. Gabbro.] 



such, and it is impossible to decide the species. It is assumed that the red feldspar 

 in the other portion is mainly orthoclase, but some of the smaller grains are twinned 

 like the albite or Carlsbad type, and may be triclinic. One section. 



Age. Cabotian. N. H. w. 



No. 1798. CLAYSTONES (?) 



"Siliceous pebbles or claystones from the Keewatin schists or slates about a mile and a half northeast of 

 Otter Creek station, Oarlton county, at the highway north of the St. Paul and Duluth railroad." 

 Ref. Anrual Report, xxii, page G. 



Meg. Oval, dark, slate-like masses, which in weathering stand out above the 

 the surrounding schists and are of darker color than these schists on fresh fracture. 

 They may be pebbles or they may be concretions. No section. 



Age. Archean (Keewatin). u. s. G. 



No. 1799. GABBRO. 



Duluth, at No. 1013 Michigan street. 

 Ref. Annual Report, xxii, page 6. 



Meg. " Pebbles of disintegration." 



Mic. The feldspar grains are fractured as by dynamic action, and the most of 

 the augite is changed to umlite; otherwise this seems to be a normal gabbro of the 

 region. One section. 



Age. Cabotian. 



Remark. There is a thickness of twenty-live feet, visible in the bluff by the 

 street, in which the gabbro is in a state of pebbly disintegration. It may have been 

 caused, as indicated by the condition of this slide, by crushing under dynamic strain, 

 rather than by normal disintegration. N. H. w. 



No. 1801. GABBRO. 



Duluth; taken at a point back of Rice's point, where the grand boulevard (running approximately on the 

 upper beach) crosses a-small creek, on the west side of the creek. 

 Ref. Annual Report, xxii, page 6. 



Meg. The rock has a gneissic structure. 



Mic. There is preserved an ophitic structure, and the feldspar, probably 

 labradorite, is well preserved. The augite contains many small grains of magnetite, 

 which also forms larger grains elsewhere. Sphene in small grains is distributed 

 generally, and zoisite in aggregated fine grains is general in the feldspar. A small 

 amount of hornblende has taken the place of some of the augite. One section. 



Age. Cabotian. N. H. w. 



No. 1802. GABBRO. 



Duluth; same place as the last. 

 Ref. Annual Report, xxii, page 6. 



Meg. Spotted with dark green, and with red-weathering. 



