2 PROCEEDINGS OF THE NATIONAL, MUSEUM. vol. (jG. 



and help in interpreting the various phenomena and in the micro- 

 scopic work, Dr. Henry S. Washington for courteously reviewing that 

 part of the discussion which deals with the quantitative classification ^ 

 and Dr. Edgar T. Wherry for carefully reading and editing the 

 manusci-ipt. To Harry Warner and Frank Reid, preparators for the 

 National Museum and Geological Survey, I am indebted for the 

 skillful preparation of the numerous thin sections required. Finally, 

 I would acknowledge as the work of J. S. Olmstead, photographer 

 of the National Museum, the excellent natural-size photographs her© 

 reproduced. The photomicrographs I myself took in the Depart- 

 ment of Geology laboratory. 



LOCATION. 



The locality in question is a quarry opened in the diabase for " trap 

 rock" which is crushed and sold for road making, and is alongside the 

 right of way of the Washington & Old Dominion Electric Railway just 

 east of Goose Ci-eek, about 6,400 meters (4 miles) southeast of Leesburg. 

 The quarry is about 800 meters (^mile) northwest of Belmont Park 

 Station. 



GENERAL RELATIONSHIPS. 



No attempt was made to work out any areal geology except to casu- 

 ally examine a few outcrops in the general vicinity. The locality is 

 within the Harpers Ferry Quadrangle described by Keith. The out- 

 crop of the diabase as shown on Keith's map is extremely irregular 

 with a maximum width at the south of the quadrangle of about 6,400 

 meters (4 miles). The section shown as crossing the diabase sheet 

 about 8,000 meters (5 miles) to the south of the quarry indicates an 

 intrusive mass of sill-like form, in general conformable with the bed- 

 ding, having a thickness computed from the section of some 750 meters 

 (2,400 feet) . The quarry here described lies within a few dozen meters 

 of the eastern edge of the outcrop and hence is presumably practically 

 at the base of the sill. The ridge on which the quanT is situated is 

 entirely composed of the trappean rock. The next ridge to the east 

 is seen, where cut through for the railroad, to consist of baked and 

 mottled Triassic shale, while the intervening vale is devoid of expos- 

 ures. Keith gives a brief description of the diabase^ stating that it 

 is intrusive \vith a maximum width of possibly 250 meters (800 feet) . 

 If it actually is as thin as this the base must be somewhat flatter than 

 shown on his section. The Triassic rocks of Virginia are now being 

 studied for a report to be published by the Virginia Geological Survey. 

 A preliminary paper on the diabases, including the diabase pegmatites 



'Arthur Keith, Harpers Ferry Folio, Folio 10. U. S. Geol. Surv. 



