2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.66 



opinions throughout the preparation of the paper and to Dr. Edgar 

 T. Wherry for kindly reviewing the manuscript and offering numer- 

 ous helpful criticisms. 



LOCALITY 



The locality described is a quarry at the north side of the Wash- 

 ington & Old Dominion Electric Eailway a short distance east of 

 Leesburg station. The main pit now being worked is about 80 

 meters long by 30 meters wide and about 30 meters deep. The rock 

 is the limestone conglomerate, known as Potomac marble, which 

 frequently occurs at the western border of the Triassic area, and 

 consists of limestone fragments in a calcareous matrix, the product 

 being used as lime, mainly for agricultural purposes. The location 

 is about three miles northwest of the previously described Goose 

 Creek diabase quarry and is immediately above the roof of the same 

 intrusive diabase sill. 



GENERAL RELATIONS 



The bottom of the quarry is believed to be, at most, only a few 

 meters from the roof of the large sill, the contact of which is re- 

 ported to have been encountered in an adjoining quarry which is 

 now filled with water. Near the bottom of the east wall of the pit 

 two dikes of basalt are exposed, which are doubtless apophyses 

 of the main igneous mass. The exact relations of the dikes are 

 not clear mainly because of a fault which is exposed here and 

 which has greatly fractured the limestone, locally largely replaced 

 by diopside, but they seem to dip at a low angle to the east toward 

 the sill, the roof of which apparently dips west. The rock quarried 

 from the eastern side of the quarry is much harder than the rest, 

 owing to being higher in silica, and is used mainly for road " metal." 

 The exact attitude of the faulting likewise could not be made out 

 but both the basalt and the silicated limestone have been involved 

 in crushing movements. The large amount of diopsidation of the 

 limestone adjacent to the dikes is probably not due to the fact that 

 the dikes are intruded here but rather to the fact that the fissures 

 along which the dikes were intruded have been reopened giving 

 a channel which carried the heated solutions emanating from the 

 sill. Although the attitudes of the large masses of diopside and 

 diopside garnet rock are not clear, the manner of their formation 

 is indicated by certain smaller altered zones along smaller frac- 

 tures where the replacement of the limestone can be studied in de- 

 tail. Some of these are well exposed in the north wall of the quarry, 

 and a typical cross section of one of them, illustrated in plate 1, 

 shows the principal features of the replacement. These are re- 



