HUDSON AND MOHAWK DISTRICT. 261 



The composiiion is not constant : the lime varies from four to six per cent, and the mag- 

 nesia from a trace to the amoiml given in the above analysis, whicli may be regarded as 

 the maximum quantity. The analysis by hydrofluoric acid gives a result which does not 

 difler materially from the above. The amount whicli is credited to the silicates may be 

 regarded as nearly pure silex, as this amount is removed when it is submitted to tiie action 

 of hydrofluoric acid, wliich acts upon the silex. 



This clay extends into the Mohawk valley, and forms an admirable basis for alluvial 

 flats which border the river. Its composition in Montgomery county gives a result some- 

 what different from the analysis above. We obtained, for example, from a specimen at 

 Fonda, 



Water of absorption and vegetable matter 9-75 



Silex 71-92 



Peroxide of iron and alumina 14-98 



Carbonate of lime I •la 



Magnesia 0-70 



99-10 



Potash w^as not sought for. 



It appears from numerous examinations which we have made, that clays contain more 

 or less vegetable matter ; the}' all blacken previous to ignition, and give off the odor of 

 burning vegetables. 



A still greater difference of composiiion exists in the claj-stones of this formation ; thus, 

 they contain 



Water of absorption 6-2S 



Organic matter 1-70 



Silex 30-88 



Peroxide of iron and alumina 9-42 



Carbonate of lime 50-98 



Magnesia 0-22 



■' -■ 99-48 



The clay-stones or concretions may be regarded as recent productions, inasmuch as 

 many appear to be unfinished. They increase in size by accession of matter upon the 

 outside ; and as tliey contain a much larger amount of lime than the adjacent layers of 

 clay, this addition seems to be taken from the particles of lime disseminated through the 

 mass. Their mode of formation is instructive, as it illustrates the manner in which septaria 

 have been formed in the slates and shales of the Erie division of the New- York rocks. 



Concretions are not constant in composition ; though from the analyses which we fixid 

 in different authors, the lime is more constant than the other elements. To illustrate this 

 fact, we quote here the analyses of concretions from several localities in Vermont, by 

 Prof. Adams : 



