472 WARREN AND POWERS DIAMOND HILL-CUMBERLAND DISTRICT 



thin films of red iron oxide. Iron oxide films a millimeter or more thick 

 sometimes cover the quartz crystals in the vugs, and there is sometimes a 

 segregation of the same along the sides of the veins in what was originally 

 fragments of the felsite. There is, besides, a more or less general stain- 

 ing of the silicified felsite inclosed between the veins. The iron oxides 

 are in part turgite, in part geothite, with perhaps some hematite and 

 limonite. Under the microscope the quartz is typical vein quartz, often 

 containing minute black specks or fine dark dust. Every stage in the 

 replacement of the felsite by the quartz may be seen. Sericite and kaolin 

 appear as products in the replacement. The iron oxide is probably re- 

 sidual from the felsite, although some of it may have been introduced by 

 the siliceous waters. Sulphides are not now found, but were doubtless 

 present, as they are found in other quartz veins in the neighborhood, and 

 by their oxidation may have furnished part of the iron oxides. Some 

 chalcedonic and opaline silica are found in fissures and lining pockets on 

 the top of the hill, and jasper agate is reported by Kunz^^ as occurring on 

 the hill in comparatively lai'ge amounts. At the southern end of the 

 hill, where a railroad cut has been made, a coarse granite of the biotite 

 type occurs in contact with the felsite. This has also been almost com- 

 pletely silicified, so much so that the actual contact with the felsite is 

 obscured. A number of quartz veins run out for great distances into the 

 granite at the eastern base of the hill, as well as into the felsite on the 

 western side. 



Another feature of mineralogical interest is the frequent occurrence in 

 the quartz of casts of square, rectangular, or tabular outlines. These are 

 surrounded by combs of quartz crystals and an incrustation of the same 

 occurs on the walls of the casts. The casts will measure from one-quarter 

 to three-quarters of an inch on a side. All of these have been identified 

 as casts of barite crystals by Prof. Charles Palache, of Cambridge. 



The injection of the quartz at Diamond Hill evidently took place after 

 the granitic intrusion in Middle Carboniferous time. Toward the end of 

 the mountain-building movements a north-south faulting occurred along 

 the western edge of Diamond Hill. This faulting was accompanied by a 

 brecciation of the felsite and the development of north-south fissures. 

 Tip these fissures came hot silica-bearing solutions, presumably the pneu- 

 matolitic fraction of the adjacent granite magma. The quartz, then, rep- 

 resents the dying phase of igneous activity in this locality. The quartz 

 replaced the felsite where the brecciation was greatest and forced its way 

 through the adjoining rock. It is very noticeable that the silicification 

 was largely confined to the felsite, which was more easily fractured than 



"Mln. Res. U. S., 1893-1894, p. 749. 



