ART. 28 MINERALOGY OF TEIASSIC LIMESTONE SHANNON 17 



place extensively without obvious eifect. In the examples which, 

 because of their economic importance have received much careful 

 study the solutions have produced, in their early stages, when en- 

 closed in limestone, garnet zones, while at greater distance or in 

 different surroundings and under other conditions they gave rise to 

 metalliferous veins. There is no real difference between metallifer- 

 ous veins, such as are widely known, on the one hand and the 

 less conspicuous zeolite-bearing veins arising from basaltic rocks on 

 the other hand, and ore minerals are frequently noted in association 

 with the zeolites while zeolites are not infrequent in association with 

 valuable ores. 



At Leesburg the solutions emanating from the crystallizing dia- 

 base have penetrated the limestone, after some reaction with the 

 traversed igneous rock, and have followed fissures replacing the lime- 

 stone adjacent to these fissures by lime-silicates. The most abundant 

 mineral is diopside, followed by garnet and vesuvianite and later 

 serpentine, followed by magnetite. The diopside replaced, first, the 

 porous material of the calcareous sand groundmass and, later, the 

 more porous of two kinds of marble making up the pebbles of the 

 conglomerate, leaving even small pebbles of the other less porous 

 marble isolated in a diopside groundmass. The serpentine-forming 

 solutions coming later penetrated these residual pebbles of limestone, 

 coloring them with disseminated flakes of serpentine and ^irround- 

 ing them with a serpentine crust. Tlie magnetite moreover replaced 

 these serpentinized pebbles of limestone in preference to the pre- 

 viously form.ed diopside and garnet of the matrix. Advocates of 

 the origin of lime-silicate zones by decrease in volume and recrystal- 

 lization might maintain that the presence of clay and sand as im- 

 purities in the groundmass Avas the controlling factor in this localiza- 

 tion. The attitude of the silicates along a fissure in unaltered rock 

 shows, however, that whatever agency created them traveled along, 

 and confined itself to the immediate vicinity of, the fissure. More- 

 over, the structure of the conglomerate is retained, showing that 

 there has been no considerable decrease in volume or concentration 

 of impurities. The process has apparently been entirely metaso- 

 matic and volume for volume without any alteration or loss of 

 structure. It is conceivable from a study of the specimen illustrated 

 in plate 1 that, assuming the limestones to have been a bedded series 

 instead of the conglomerate, the buff marble might have been com- 

 pletely converted to silicate rock while the gray and white marble 

 remained unchanged or was replaced by magnetite, it being as- 

 sumed, of course, that the supply of replacing solutions was adequate 

 and not limited as in the illustrated specimen. On the eastern side 



