HUDSON AND MOHAWK DISTRICT. 261 



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

 nesia from a trace to the amount given in the above analysis, which may be regarded as 

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

 differ materially from the above. The amount which is credited to the silicates may be 

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

 of hydrofluoric acid, which acts upon the silex. 



This clay extei^ls 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 1*75 



Magnesia 0'70 



99-10 



Potash was not sought for. 



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

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

 burning vegetables. 



A still greater difference of composition exists in the clay-stones of this formation ; thus, 

 they contain 



Water of absorption 6*28 



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 they 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 find 

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

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

 Prof. Adams : 



