18 PROCEEDINGS OF THE NATIONAL, MUSEUM vol.66 



of the quarr}', where the action was more intense, all parts of the 

 limestone have finally succumbed to the replacement. 



It seems probable that the extent of the metamorphic replacement 

 of the limestone at Leesburg is more or less coincident with and 

 dependent upon the abundance of water in the underlying sill and 

 upon the formation of pegmatites and acid differentiates in the sill. 

 This mass thus behaved more or less as an abyssal chamber, retaining 

 its volatile emanations and concentrating them in differentiates. 



Characteristic of a somewhat later phase of the activity of the 

 solutions are the low temperature veins, corresponding to the zeolite 

 veins at Goose Creek, which are described below as containing dato- 

 lite and calcite with less apophyllite, diopside, and barite. 



LOW-TEMPER ATURB VEINS 



Under this heading are considered narrow fractures in the lime- 

 stone containing fillings of calcite or, more frequently, clatolite, and 

 having numerous open spaces lined with datolite, calcite, and less 

 of a peculiar form of diopside, apophyllite, and barite. These veins 

 average only about 2 centimeters in width, although they widen out 

 in places to 8 or more cm. with open centers. The veins fill open 

 cracks which are apparently feeble breaks of practically no dis- 

 placement. The open space which they have filled may in part be 

 due to solution of the limestone along the break. The adjacent lime- 

 stone is not conspicuously altered. These veins are considered to 

 represent the material deposited from solution by emanations from 

 the underlying diabase in the same manner that datolite with zeo- 

 lites and prehnite were deposited in the veins in the diabase. The 

 source for these vein minerals is thus the same as that of the ma- 

 terials added to the limestone to form the replacements composed of 

 lime-silicate minerals and magnetite. The datolite-calcite bearing 

 veins are considered to represent a slightly later phase of deposition, 

 marked by lower temperature and perhaps pressure, indicated by 

 the fact that they cut the lime silicate rock but have produced no 

 notable alteration where they have intersected the original lime- 

 stone. The parent cracks which controlled the lime-silicate depo- 

 sition described above are filled with calcite and datolite occurring 

 in the cracks along which the basalt has been hydrothermally 

 altered. In general the deposition of the later veins followed new 

 fractures, but the veins are linked to the high temperature replace- 

 ments by a number of features in common. Calcite veins cut the 

 lime silicate body on the east side of the quarry and one of these 

 had a central filling of chalcedony like that observed in thin section 

 in lime silicate rock. Moreover, diopside, the most abundant product 

 of the lime silicate replacement, occurs as a true vein mineral inti- 

 mately associated Avith the datolite. 



