PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 513 



Slate.] 



to 725 are samples. These sills, as far as examined, always show a much finer and 

 usually more altered lower side, and in the field there is some difficulty in distinguish- 

 ing this lower portion of a sill from the immediately underlying, almost black, slaty 

 rocks which have been somewhat altered by the diabase contact. There is, however, 

 no "transition " between the diabase and the slate, as is seen by an examination of the 

 thin sections, and usually one who is accustomed to these rocks can distinguish the 

 two within an inch of the contact; and frequently the exact contact line can be 

 determined. 



Quite, a number of specimens from the diabase sills (Logan sills) of the 

 Animikie in Cook county have been collected. Among these are the following from 

 the vicinity of (iunflint lake: Nos. 308, 709, 710, 717 to 725. u. s. o. 



No. 726! SLATE. 



A foot lower than No. 725. Same locality as No. 721. 



Jiff. Annual Report, ix, page 81; Annual Report, x, page 87. 



Mnj. A fine-grained, earthy, yellowish-gray slate. A little pyrite is present. 



Mic. The most noticeable feature of the section under a low power is the 

 presence of numbers of dark spots which are of rather irregular, though approxi- 

 mately circular, outline. They are fairly uniform in size and average a little less 

 than .1 millimeter in diameter. Under a high power these spots are seen to be 

 more irregular in outline, and they are not very sharply mai'ked off from the rest of 

 the rock. The section is composed of a very fine-grained aggregate of quartz, possibly 

 some feI<l*i.H/r, mitxcorife, rli/arife, /ii/rite, iittiyiiefifr, and probably some carbonaceous 

 material. The iron ores are the largest grains in the rock. The dark spots are seen 

 to be aggregations of the opaque minerals of the rock and chlorite, quartz and 

 mnscovite being in small amount or lacking entirely. Much of each spot is made 

 up of minute black specks which are quite probably of carbonaceous material. 



One section examined. 



Age. Animikie. u. s. G. 



No. 727. SLATE. 



Four feet lower than No. 726. Same locality as No. 721. 



Kef. Annual Report, ix, pages 81, 82; Annual Report, x, page 87. 



Met/. A slate similar to No. 726, but darker colored. 



Mir. The section is very similar to No. 726, but is even finer grained and does 

 not show as much HI unco rife. Some yellow grains which appear to be e.j>i<li>fe are 

 present. The black spots are numerous, but average fully twice the size of those in 

 the section of No. 726. 



One section examined. 



Age. Animikie. 



34 



