MINERALOGY AND PETROGRAPHY OF TRIASSIC LIME- 

 STONE CONGLOMERATE METAMORPHOSED BY IN- 

 TRUSIVE DIABASE AT LEESBURG, VIRGINIA 



By Earl V. Shannon 

 Assistant Cnrator of Geology, United States National Museum 



INTRODUCTION 



The present article is intended to follow a preceding much length- 

 ier paper on Triassic diabase at Goose Creek, Virginia.^ In that 

 paper the diabase, which forms an intrusive sill-like mass several 

 hundred meters in thickness, is described in detail, and it was con- 

 cluded that certain secondary minerals, among them datolite, 

 prehnite, apophyllite, and certain zeolites, were deposited by mag- 

 matic waters expelled by the diabase magma at the end of its con- 

 solidation. Various hydrothermal effects of the magmatic solutions 

 upon the consolidated diabase were also considered. The following 

 description considers the case where these magmatic solutions, 

 emanating from the crystallizing diabase, ascended along fissures 

 in the overlying limestone and the alteration of the limestone and 

 the secondary minerals deposited, both as fillings of open cavities 

 and by metasomatic replacement of the limestone itself, are de- 

 scribed in detail. 



The quarry was visited at various times with several other min- 

 eralogists, namely, Frank L. Hess, Esper S. Larsen, Clarence S. 

 Ross, Waldemar T. Schaller, and Ralph W. G. Wyckoff, to all of 

 whom I am deeply indebted for valuable assistance and advice.^ I 

 would especially express my thanks to Doctor Ross for help and 



1 Earl V. Shannon. Mineralogy and Petrography of intnisive Triassic diabase at Goose 

 Creek, Loudoun County, Virginia. Proc. U. S. National Museum, vol. 66, pp. 1-86, 1924. 



^ Since this paper was written and following the December, 1923, meeting of the Geo- 

 logical Society of America and the Mineralogical Society of America in Washington, a 

 field trip was held to this locality under direction of the writer, which was attended by 

 numerous other scientists of national and international repute. 



No. 2565.-PROCEEDINGS U. S. NATIONAL MUSEUM. VOL. 66. ART. 28. 

 9098—26 1 



