PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 119 



Diabase.] 



There are no feldspathic or augitic fragments that can be detected, and judging 

 from the persistent endurance of the minute feldspar microlites, at least in form, in 

 the lavas and all the diabases of the series, it is necessary to conclude that they never 

 existed in this rock. An abundant cement of calcite is distinguished by its iridescent 

 polarization. 



Ordinary erosion and sedimentation could hardly produce such a rock, and 

 although it has a slaty or bedded structure which suggests sedimentary action, it is 

 probable that the ocean had but little to do with its origination. Its dip is coinci- 

 dent with that of the lavas and amygdaloids of the immediate vicinity. Its strati- 

 form arrangement may have been the product of oceanic forces in spreading out the 

 debris of volcanic ejection. 



Three sections examined. 



Age. Cabotian. 



Remarks. Although some of the former numbers of this series (Nos. 8A, 8B, 

 7B) have already been described as probably of tuffaceous origin, this rock is the 

 first which has been met with presenting positive characters of that kind. Indeed, 

 it is the first positively identified tuff in the so-called'Keweenawan. N. H. w. 



No. 18. DIABASE (with olivine). 



Duluth. East of the Brewery creek, and east of Minnesota point, at the lake shore. Over this comes 

 down a little creek; this rock extends perhaps 300 feet, nearly into the bite of the next little bay, and at the 

 eastern limit of the exposure it has a dip easterly of 26. 



Ref. Annual Report, ix, pages 14, 16; Annual Report, x, page 36. 



Meg. A much decayed, rusty-green, finely-granular, massive but remotely- 

 jointed rock. On the weathered surfaces it is finely pitted by the loss of some min- 

 eral, apparently olivine. In other places it is amygdaloidal, and in others it crumbles 

 like a rotted shale. It appears, in the field relations, like a surface flow of diabase. 

 Its weight indicates a considerable percentage of magnetite. 



Mic. Microlitic feldspars are frequently reddish, from ferruginous oxidation; 

 no coarser feldspars present. 



Augite shows, though much altered and in general without its original forms, 

 occasionally its ophitic relation to the feldspars. 



Olivine, though conspicuous in the section, is changed to an almost isotropic 

 chloritic substance, yet which sometimes is finely fibrous, a fact which is seen on 

 lowering the lower nicol. These grains are the most conspicuous in the slide. They 

 show, by the dark lines of ferruginous matter, the original cleavages of the olivine. 



Quartz is a product of alteration, sometimes arranged radially, surrounding 

 chloritic masses. 



Magnetite is usually plainly of secondary origin, and is disseminated widely. 



One section examined. 



