524 NEW YOKK STATE MUSEUM 



silicates are probably tbe most important form of its occurrence in 

 clay, and are represented by tbe minerals, mica, hornblende, cblorite 

 and pyroxene. Tbese are scaly minerals containing from 15^-25^ 

 of magnesia. Mica is a very common constituent of many clays, 

 and its shining scales easily render it recognizable. Chlorite scales 

 may be present in many clays, and if in abundance color the clay 

 green. Hornblende also is not an uncommon constituent, and 

 specially present in clays derived from rocks of very basic composi- 

 tion, that is, those with a low silica percentage. Indeed the de- 

 composition of hornblende may give rise to a hydrous aluminum 

 silicate, which is highly colored by iron, the product therefore being 

 a ferruginous clay. (G. P. Merrill's Rocks, rock-weathering and 

 soils, p. 21) 



Dolomite, the double carbonate of lime and magnesia., may be 

 a source of magnesia as well as of lime in clay. 



Magnesium sulfate, or Epsom salts, occurs sparingly in clays, but 

 when present may give rise to the formation of a white coating on 

 the surface of the ware. It is commonly found in those clays where 

 sulfuric acid, set free by the decomposition of pyrite, has attacked 

 magnesium carbonates. The presence of this salt can frequently be 

 detected by the bitter taste which it imparts to the clay. 



The chemical effects of magnesia in clays are probably similar to 

 those produced by lime. This is not tO' be taken as absolutely cer- 

 tain, for magnesia is present in most clays in such small amounts 

 as to make its exact action uncertain. 



The range of the percentage of magnesia in the different clays, 

 deduced from the analyses given at the end of this report, is as 

 follows: 



Quality Min. Max. Aver. 



Brick clays .02 11.03 2.66 



Pottery clays .05 4.80 .85 



Fire clays . .02 6 . 25 . 513 



Kaolins tr 2.42 .223 



