72 GEOLOGY OF OHIO. 



large variety of other minerals. The most important of these associated 

 minerals is quartz, or free silica, or sand, as it is variously termed. It is 

 found in all clays, to some extent. Even the purest kaolins contain small 

 fractions of a per cent, of it, and on the other extreme it is hard to say 

 where a sandy clay ends and an argillaceous sandstone or freestone begins.^ 

 There is a great difference in the size of. the grains of quartz, some clays 

 being very coarse and others are so very fine grained that it becomes im- 

 possible to separate the sand from the clay by any mechanical process, 

 like washing in water. 



Feldspar and mica are found in almost all clays. Sometimes the 

 mica can be plainly discerned by the eye. Feldspar cannot be distin- 

 guished from sand by such means, but its presence is highly probable in 

 many clays. Both feldspar and mica contain the alkalies in combination 

 with silica and alumina. Hence we can most easily understand the pres- 

 ence of the alkalies in a chemical analysis of clays, when we believe them 

 to exist in these minerals. It is not possible to imagine free alkalies exist- 

 ing in a mineral which owes its origin to slow deposition in water and 

 the presence of some comparatively insoluble silicates, like feldspar or 

 mica, seems necessary to explain their continual presence. 



In this connection, the question arises, how can feldspars remain un- 

 decomposed or unkaolinized during all the exposure to water and air 

 which must have been their lot. Possibly the formation of kaolin on the 

 surface of a grain of feldspar serves as a shield to shed water from the 

 inside of the grain and possibly the retention of the feldspar in the mass 

 of tough and plastic clay protects it from continual attack by the elements. 

 It seems likely that the mica minerals contribute more largely to the pot- 

 ash of clays than the feldspars, as they are very much less affected by the 

 agencies of decomposition. 



The oxides and other compounds of irons are an almost invariable 

 constituent of clays, and probably next to the silica, they are the most 

 important. The oxides, both as sesquioxide and protoxide, free or hy- 

 drated,are the commonest forms, but the carbonates of iron are not un- 

 common, and sulphides are an occasional and very injurious impurity. 

 Possibly iron combined as silicate is sometimes present. 



Iron is the great coloring agent of clays. The tints vary from the 

 lightest buff to strong . cherry red and from drab to blue or green. The 

 amount of coloration is not proportional to the amount of iron, however, 

 as perfectly white china clays have been found in analysis to contain over 

 one per cent, of iron, and other clays containing this amount might show a 

 strong yellow or pink color. The effect of the iron on the color of a clay 

 is much increased by the burning process, and the colors produced in 

 burning vary from cream color to perfectly black— almost all tints being 

 represented, but the reds, browns and greens predominating. The condition ' 

 of the iron has much to do with its coloring power. If it is disseminated 



