ART. 28 MINERALOGY OF TRIASSIC LIMESTONE SHANNON 3 



garcled as having originated by the hydrothermal replacement of 

 the limestone and some of them are persistent to a considerable 

 distance from the diabase mass. The minerals thus developed in 

 the limestone are those typical of what are commonly called con- 

 tact metamorphic deposits in limestone. Inasmuch as the replace- 

 ment is clearly a result of the action of heated solutions, or possibly 

 aqueous vapors, on the walls of the fissure, the conception of con- 

 tact metamorphism is not greatly emphasized here, the minerals 

 being described instead as high temperature hydrothermal replace- 

 ments. The minerals occurring in this manner include diopside, 

 garnet, magnetite, serpentine, wollastonite, xonotlite, and probably 

 thaumasite. The other class of secondary minerals, regarded as 

 probably having originated at a somewhat later period marked 

 by a considerably lower temperature, occurs as crystals and fillings 

 of cracks and open spaces along slight fissures in limestone. The 

 limestone of the walls of these fissures is not greatly altered. De- 

 posits of this class include datolite, calcite, diopside, apophyllite, 

 and barite; probably anhydrite was also among these. 



THE LIMESTONE CONGLOMERATE 



Little that is original can be added regarding the limestone which 

 is quarried. It is Triassic limestone conglomerate which is com- 

 monly known as " Potomac marble " and is made up of fragments 

 of limestone of various sizes and colors in a matrix of calcareous 

 sand, the average tone of the rock as a whole being light gray to 

 almost white. Considering its heterogeneous origin the conglom- 

 erate is unusually low in quartz and other impurities. Keith ^ gives 

 the following description of the formation: 



The limestone conglomerate is made up of worn pebbles of limestone of 

 various colors, usually blue, interbeddecl in a reddish calcareous matrix. 

 Rarely pebbles of slate and gray sandstone also occur with those of limestone. 

 The pebbles were deposited in their matrix in a very irregular manner and in 

 sharply limited areas. The areas of conglomerate point off into the sand- 

 stone like wedges, their form being due either to thinning out away from shore 

 or to subsequent cutting off by faults. From these masses of limestone peb- 

 bles it is inferred that a large body of limestone was exposed to erosion and 

 that from its fragments were produced the worn pebbles. The conglomerate 

 being coarse, it was probably laid down by strong currents or waves along 

 a shore, and is therefore apparently a beach deposit. 



Doctor Merrill gives the following account of the formation:* 

 The only true conglomerate or breccia marble that has ever been utilized to 

 any extent in the United States is found near Point of Rocks, Frederick County, 



3 Arthur Keith. Geol. Atlas U. S., U. S. Geol. Survey, Harpers Ferry Folio. Folio 10, 

 p. 3, 1894. 



* George P. Merrill. Stones for Building and Decoration. New York, 1891, pp. 92-93. 



