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 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__ 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 than the other elements. To illustrate this 
fact, we quote here the analyses of concretions from several localities in Vermont, by 
Prof. Adams: 
