MINERALS OF MARYLAND. 105 



river between Rockville and the mouth of Seneca creek. The 

 principal rocks of this division are granite, gneiss, and mica 

 slate in all their varieties, besides hornblend rocks, primary 

 limestones, serpentine primary argiilite, talcose and chlorite 

 slates, &.C. 



Quartz is of course abundant, and although it frequently 

 occurs in crystals, fine specimens are rarely obtained. The 

 largest crystals are found in the part of the granite and gneiss 

 region, situated between the Patapsco river and the head waters 

 of the Little Patuxent. Mr. Gilmor* mentions a fragment 

 of one weighing nine pounds. These crystals are mostly of 

 the smoky variety and often only semi-transparent. Trans- 

 parent and limpid crystals have also been found in this and 

 other localities. 



Chalcedony^ jasper^ hornsionc, and agate exist in several 

 places, and mostly in connection with the hornblend and ser- 

 pentine rocks ; good specimens of the two former are frequently 

 found, but the finest chalcedony occurs in a primary limestone, 

 15 miles north from Baltimore; it has a smooth uniform aspect, 

 a beautiful sky-blue colour, and is highly translucent. Fetid 

 quartz also occurs in the primary limestone. 



The felspar of this district is mostly of the ordinary kind, and 

 in some localities constitutes almost the whole of the granite. 

 On the Baltimore and Ohio rail road, 9 or 10 miles from this 

 city, large veins of a granite of this kind are protruded from 

 below into both the gneiss and hornblend rocks. Occasionally 

 specimens of the felspar have a regular crystalline structure, 

 so that laminse of several inches in length may be obtained by 

 cleavage; the colour is mostly dull white or yellowish white 

 and salmon, but sometimes greenish. Mr. Gilmor notices 

 crystals of felspar in the gneiss on Jones' fulls. 



Compact felspar^ embracing small imperfect crystals of 

 quartz and specks of specular oxide of iron, form a rock that 

 may be ranked with the weissteiti of the Germans. It occurs 

 just where the Patapsco passes out of our primary division. 



Afi aggregate of quartz and felspar, both white and crystal- 

 line, occurs on Jones' falls, 8 miles from Baltimore, containing 

 minute prisms of tourmaline. It lies on the edge of the lime- 



* Bruce's Mineralogical Journal, vol. i. p. 323. 



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