322 GODO'N'S MINERALOGICAL OBSERVATIONS, &C 



but it appears to be only an intimate mixture of quartz, am- 

 phibol, mica, and talc, which shows itself by its green colour. 

 This rock often includes the sulphureted magnetic iron. The tint 

 of this pyrites resembling that of copper pyrites, and light spots 

 even of carbonate of copper, indicate that this mineral con- 

 ceals a small proportion of copper. This rock, as the former, 

 is crossed in several directions by veins of quartz, sometimes 

 more than two feet thick. A distinct stratification is not ob- 

 served in it; it is divided into polyhedral fragments. 



In continuing up the Potomac, at a little distance from 

 George-town, gneiss is again found, analogous in its nature to 

 that already described. The inclination of these new strata, 

 appears to be the same, but their direction somewhat nearer to 

 that of the North and South. This gneiss shows some veins of 

 white felspar, mixed with a mica of a greenish white, but the 

 opaque white quartz, exists in numerous, and powerful veins. 

 This quartz is sterile in metallic substances; some signs of oxidu- 

 lated iron only and of magnetic pyrites are found in it. This 

 quartz is sometimes accompanied by the cldoritic talc, and 

 pretty often includes the tourmalin in small acicular crystals. 

 Sometimes the quartz presents a large surface covered with a 

 crust of a fine black substance ; at first sight, this matter would 

 be taken for manganese, but by attentive observation, it ap- 

 pears to be nothing more than the substance of tourmalin 

 itself, in a state of confused crystallization. 



You go up the Potomac river to the little falls, four miles 

 above Washington, without finding any remarkable change in 

 the constitution of the gneiss. This rock also crosses the river, 

 and you may observe, on both the banks, the same disposition 

 in the strata, and the same characters. The vegetable earth, 

 which covers the tops of steep hills on the left side, is nothing 

 but the gneiss itself in a state of decomposition, which is capa- 

 ble of being turned up by the plough ; and the fields are 

 covered by numerous fragments of quartz, which have suffered 

 no alteration. 



The two beds of gneiss which are distinctly observed, on the 

 right bank of Potomac river, and which are separated on this 

 side by the Amphibolic rock, appear to be re-united on the left 



