349 



they might be usefully employed as a delicate water ce- 

 ment, when properly prepared. 



Over this heavy formation of marly clay, and likewise 

 constituting the surface of the extensive and elevated plain 

 which forms so conspicuous a feature in the scenery be- 

 tween the city of Albany and the Mohawk valley, is spread 

 out a thick deposit of yellow ferruginous sand. It leaves, 

 however, in many places, denuded portions of the clay 

 protruding far above the soil; and also, fills up those deep 

 grooved trenches, which have been torn out by the rush- 

 ing waters loaded with detrital matter from the north, in 

 such a manner that it can only be seen to advantage along 

 the deep cuttings, and in the banks of the numerous 

 streams that discharge their waters into those of the prin- 

 cipal rivers. 



This sand is very unevenly spread over the surface of 

 the clay, so that it becomes exceedingly difficult to form a 

 just conception of its thickness, it having been drifted and 

 blown about into variously shaped knobs or hills, some of 

 which attain an elevation some distance above the general 

 level of the plain; about thirty feet, however, I should 

 consider in all probability a fair estimation of its greatest 

 depth. 



This yellow sand is chiefly composed of silicious parti- 

 cles having an external coating of oxide of iron, which 

 gives to it that deep ochreous appearance which it every- 

 where presents, when not exposed to the direct influences 

 of atmospheric action; small fragments of magnetic iron 

 in the form of sand, with feldspar and mica, are commonly 

 found disseminated through it, and more rarely garnets 

 and pyroxene. 



After heavy showers of rain, the surfaces of roads run 

 ning through this plain are literally strewed with small 

 concretions of this brown oxide of iron, these, upon be 

 mg washed into the low marshy depressions which are 

 every where abundant, unite themselves together by the 



