ART. 2. PETROLOGY AT GOOSE CEEEK SHANNON. 9 



As a necessary complement of the strong divisions of the rock by 

 both north-south joints and east-west fractures there developed nu- 

 merous cracks of various attitudes, mainly at a small inclination to 

 the horizontal, connecting the steeply dipping breaks. These were 

 relieved of the necessity of accommodating movement after the for- 

 mation of the major fractures and were consequently readily healed 

 by minerals deposited from solution. Examination of the walls of 

 the quarry is more or less unsatisfactory, a large part of the standing 

 rock being bounded either by north-south joints or east-west frac- 

 tures in which the structure of the rock is concealed by diabantite or 

 hornblende coatings. Rock thrown down by blasting furnishes the 

 best cross sections of the blocks between the veneers. These broken 

 blocks tend to split along the lines of weakness formed by the old 

 mineral-healed cracks. ' This results in the exposure of druses of va- 

 rious minerals, the drusy surface of a block uniformly coated with a 

 layer of a single mineral sometimes amounting to 10 square meters 

 or more. The original attitude of these druses was somewhat in 

 doubt until chabazite- and calcite-coated druses were seen in place 

 as horizontal connecting seams between steep north-south joints in 

 the southwest corner of the quarry. Usually the druses are occu- 

 pied by a single mineral so that they yield little paragenetic evi- 

 dence. Among the most common druse minerals are hornblende, 

 chabazite, stilbite, laumontite, and calcite. Chabazite and one vari- 

 ety of hornblende are found in no other form. In a few cases calcite 

 chabazite, and stilbite occupy the same druses, one overlying the 

 other. Most of the druse minerals are not notably different from 

 the same minerals which form the fillings of open spaces in shear 

 zones. They are described in detail below under the head of hydro- 

 thermal vein fillings. 



NORMAL DIABASE. 



Although not possessing a strictly diabasic texture, the rock making 

 up the body of the intrusion will be designated diabase, especially 

 since m_ost of the intrusive rocks of the Triassic of similar attitude 

 and composition have long been referred to in the literature as 

 diabase ; and to call the present intrusion a gabbro or diorite, which 

 it approaches in texture might lead to some confusion. 



The normal rock was studied in some 15 specimens and thin sec- 

 tions from various parts of the quarry and in three specimens col- 

 lected for comparison from the point where the Belmont Park road 

 joins the Leesburg Pike 2,000 meters {l{ miles) northeast of the 

 quarry. 



The rock is medium gray in the hand specimen and white feldspar 

 and greenish black pyroxene are easily distinguishable under a lens. 

 The average grain size throughout the quarry is about 1 millimeter, 

 94110—24 2 



