PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 259 



Diabase.] 



before seen, and throwing up the firm heavy beds of No. 208, at a high angle. This 

 dike is basaltic perpendicular to these beds by being cooled by them. This last 

 larger dike is only exposed near the water, and its exact contact with No. 208 is 

 invisible. It is exposed about fifty feet." (Annual Report, ix, pages 53, 54.) 



This description of the field appearances seems to show that these beds are older 

 than a series of coarse diabase dikes which cut them, and which is presumably of 

 the same date and origin as the sheet at Grand Marais. It is for that reason that 

 these beds (No. 208) are put in the Cabotian along with the " red rock " of the region. 

 The brown fine-grained rock having this number is allied to Nos. 217-219, but shows 

 little or no glass. 



If the foregoing parallelisms are correct, it is necessary to admit that before 

 the Grand Marais diabase, there was a large series of surface eruptives, both basic 

 and acid. If the Grand Marais diabase is the parallel of the Beaver Bay diabase, as 

 believed, and that is the continuation of the Duluth gabbro, as believed, it is also 

 necessary to allow an eruptive epoch which gave origin to basic surface flows at 

 Duluth prior to the advent of the gabbro of that place as well as cotemporary 

 with it. N. H. w. 



No. 209. DIABASE (with olivine). 



West of the mouth of Devil's Track river. Basaltic perpendicularly to the tilted beds of No. 208. Runs 

 nearly east and west. 



Ref. Annual Report, ix, page 54. 



Meg. A medium-grained diabase, dark gray in color. There are numerous 

 small grains of a yellowish mineral (olivine) in the rock, and these give a yellowish 

 tinge to the hand sample. 



Mir. A medium-grained olivine diabase, the olivine being quite abundant. It 

 is altered more or less to a brownish substance. The rock is comparatively fresh, 

 but still has a number of dirty greenish areas and also areas of chlorite in radiating 

 masses; perhaps these areas represent, in part at least, interstitial unindividualized 

 magma. 



One section. 



Age. Cabotian (?) D. s. o. 



No. 210. DIABASE. 



Southeast corner of sec. 8, T. 61-2 E. It forms a low exposure and is a firm, smooth-weathering rock. 

 Ref. Annual Report, ix, pages 54, 55. 



Meg. A rather fine-grained, brown, diabasic rock. It is permeated by small 

 areas of a soft greenish mineral. 



Mic. The section shows a fine-grained diabase, with large quantites of hematite. 

 The hematite is in large part secondary, filling in the areas between the feldspars 

 and replacing the augite; it also occurs in transparent films in numerous cracks. 



