QUAKTZITES OF THE HOLOCRYSTALLINE AREA. 309 



b. Orbicular quartzite, of compact and fine grain, filled with hollow groups of 



radiating quartz crystals. This rock is exposed along the western 

 edge of the Texas augen-gneiss area, and is quarried at the Poor- 

 House. It occurs also in the marble at Brooklandville. 



c. White conglomeratic quartzite of Deer creek. Among the most distinctly 



metamorphosed rocks of the eastern area is a belt of white conglom- 

 eratic sandstone in the center of Harford county, known as the 

 " Rocks of Deer creek." The band is not over four miles long and 

 from a quarter to a half a mile broad. Its hardness has caused it 

 to resist erosion, so that w^here it is cut by the stream and railroad 

 it towers up as a narrow ridge to a height of nearly three hundred 

 feet. The microscopic structure shows that this rock has suffered 

 almost complete recrystallization, although this has not always oblit- 

 erated the original pebbles. During this process, which was prob- 

 ably assisted by fumarole action, a number of new minerals were 

 abundantly developed. These are, muscovite, in continuous w^avy 

 membranes; blue cyanite in large radiating tufts; chlorite; magnetite 

 and tourmaline, garnet and rutile. This formation, which has not 

 yet been studied in any detail, lies just on the boundary between the 

 holocrystalline rocks and the semi-crystalline schists surrounding the 

 Peach Bottom slates. It cannot as yet be assigned with definiteness 

 to either area, but its position suggests that it may not impossibly 

 represent a basal conglomerate of the latter. This rock presents 

 many points of petrographical resemblance to the metamorphic sand- 

 stone of Willis mountain, Buckingham county, Virginia, described 

 by W. B. Rogers.* 



Marble (Dolomite). — The limestone of the eastern area occurs in the form 

 of highly crystalline marble. It stretches in irregular and sharply folded 

 patches through Harford, Baltimore and Howard counties, but it has not 

 been anyw'here encountered in Maryland south of the Patuxent. This mar- 

 ble is a dolomite with a varying proportion of MgCOg, whose average is 

 perhaps about 40 per cent.f It is extensively quarried as a building-stone 

 at Cockeysville, fourteen miles north of Baltimore. A very coarse-grained 

 variety, locally known as alum-stone, occurs somewhat south of this place at 

 Texas, where it is quarried for burning or for a flux. This highly crystal- 

 line marble presents a marked contrast to the finer-grained marbles of the 

 western area in having its impurities crystallized out as silicate minerals 

 instead of being intercalated as argillaceous bands. Among these accessory 

 silicates may be mentioned phlogopite, tremolite, white pyroxene, tourmaline, 

 scapolite and rutile. 



* Geology of the Virginias, 1884, p. 71. 



tFor Analyses, see Bull. U. S. Geol. Survey, no. 60, 1890, p. 1.59. 



XLVI— Bull. Geol. See. Am., Vol. 2, 1890. 



