godon's mineralogical observations, &e. 321 



been traced, we shall perhaps have some light thrown on the 

 circumstances which have produced it; and perhaps it will even 

 be possible to form some hypothesis concerning the origin ot 

 this vast deposit of sand, which is observed through the space 

 of 50 miles, in the direction from North-East to South-West,* 

 and winch appears to be much more extensively in the direc- 

 tion from East to West. 



Washington city is built on the alluvial land; but Rock- 

 Creek, which separates this capital from George-town, appears 

 to present the boundary line between the primitive and the 

 alluvial soil. 



PRIMITIVE SOIL. 



The first rock which presents itself must be considered as a 

 gneiss; its direction is nearly N. N. E. to S. S. W. Or inclining 

 about 20 degrees to the East. The substances which compose 

 it are quartz, felspar, mica, and very often talc. The mica 

 and the talc have the grey colour of lead ; but this last is 

 sometimes distinguished by the green colour ot the emerald. 

 Besides those substances, the gneiss often contains the dodecahe- 

 dral garnet, commonly in small crystals, but some are four or five 

 lines in diameter, and sulpkureted magnetic iron crystallized in 

 small cubes. This mineral sometimes exists so abundantly 

 that every part of the rock is sensible to the magnet. Con- 

 siderable veins of quartz run through the rock, without any 

 constant direction ; the veins of felspar are more rare. This 

 gneiss crosses the Potomac river, and in the opposite bank of 

 this river observes the same direction, the same inclination 

 in the strata, and the same elements in their composition. 

 This rock is generally split into tabular fragments, which are 

 employed in the construction of the foundation of buildings, 

 and in the lining of causeways. 



Immediately after the gfieiss, in going up the river, we find 

 the Amphibolic rock, (grunstein;) this rock is not uniform in 

 its composition, most frequently it is an aggregate of Amphi- 

 bol, (hornblende) and felspar, then it is nearly decomposed; 



* The primitive soil appears near Baltimore, and it is manifest in the valley through whir h 

 the Patapsco river flows. 



