The Shinumo Area, 521 



veins of asbestos occur at several horizons in proximity to the 

 contact. 



The geological occurrence of the asbestos is fully described 

 by Diller in "Mineral Eesources of the U. S. for 1907" (Diller, 

 a\ and again by the same writer in " Mineral Resources for 

 1908" (Diller, '5). The contribution of the writer of the 

 present article consists of the further data afforded by a micro- 

 scopic study of the rocks associated with the asbestos in this 

 locality. A microscopic study was made of 25 thin sections 

 cut from the limestones, the bands and nodules of serpentine, 

 and across the veins of asbestos. Aside from the serpentine 

 and asbestos no other minerals were revealed in the limestones 

 beyond the dolomitic calcite and interlocking grains of quartz 

 already described in the slides cut from the limestones of the 

 same horizon in the section on the East Wash, where the same 

 strata lie in undisturbed sedimentary contact. The texture of 

 the limestones is entirely that of marble. The serpentine of 

 the bands and nodules shows no trace of an alteration structure 

 which might indicate a derivation from pyroxene, hornblende, 

 or olivine. The slides cut across the veins of asbestos showed 

 them to be later than the serpentine in which they are usually 

 enclosed. A great number of veins of asbestos of microscopic 

 size was revealed in some of the slides where their presence 

 was unsuspected. Some of these veins were observed cutting 

 across both the serpentine and the limestone in the same slide. 



The asbestos which occurs in the larger veins is commercially 

 of high grade and the fiber is of great tensile strength. It is 

 pronounced by Diller to be the best in quality yet found any- 

 where in the United States. (Diller, 5, p. 11.) Locally the 

 crossfiber is 4 inches in length. The horizon of the larger 

 veins is confined, so far as is known, to the limestones which 

 lie beneath the lower contact of the diabase sill. The veins 

 above the contact, although more widely distributed through 

 the limestones, are usually of smaller size. The horizon below 

 the contact is not absolutely constant in stratigraphic position 

 and may lie anywhere from 3 to 15 feet below the contact. 

 The width of the veins within this horizon varies greatly from 

 place to place, so that a vein of three inches in width in one 

 locality may be represented by a zone of innumerable small 

 veins in another, but the actual continuity of the zone that 

 carries the asbestos is rarely broken. 



Conclusions. — The serpentine and asbestos occur in the 

 limestones only where these strata are invaded by the dia- 

 base sill ; where the diabase lies between shales there is no 

 development of these minerals within the invaded strata. In 

 no place in the area are they developed within the diabase 

 itself. It is therefore clear that they are a contact metamorphic 



