70 GEOLOGICAL RELATIONS OF ALABAMA CLAYS. 



fined, so (that an account of the geological relations of 

 these clays becomes a guide at once to the several va- 

 rieties, and to their geographical distribution. 



ARCHAEAN AND ALGONKIAN. 



These two formations include in Alabama all the 

 crystalline rocks of both igneous and sedimentary 

 origin. It is generally acknowledged that kaolinite, 

 which is the basis of all clays, has its origin in the de- 

 composition of the minerals composing the igneous 

 rocks, the chief kaolinite producing mineral being 

 feldspar. It is. therefore, in the area of our crystalline 

 or metamorphic rocks that we are to look for the origi- 

 nal deposits of kaolinite. More especially, it is the 

 granites, the pegmatites or graphic granites, that occur 

 the largest proportion of feldspar, and consequently 

 yield the largest proportion of kaolinite, and of the 

 granites, thepegmatites or graphic granites, occurring 

 in veins which traverse .the other crystalline rock, are 

 by far the most important in this respect. 



The clays occurring in this form have been spoken 

 of by Dr. Hies as vein clays*, and they are, as a rule, 

 very slightly plastic, for the reason that they have not 

 been subjected to the comminuting processes neces- 

 sary to develop the highest degree of plasticity. 



A belt of mica schists with frequent veins of peg- 

 matite, extends from Cleburne county and adjacent 

 parts of Randolph, through Clay and Coosa into 

 Chilton county, and in numerous places, the decay of 

 the granite veins has given rise to the formation of 

 deposits >olf kaolinite,. The other two constituents of 

 these granites, viz., quartz and mica, occur like the 

 feldspars in large masses, and thus the places which 

 produce mica in large sheets are at the same time the 



