500 NEW YORK STATE MUSEUM 



If ]\Ir Collins's theory be correct, the kaolin deposits shoulcJ 

 extend to great depths, but if the kaolinization be due to weathering, 

 then we should encounter undecomposed feldspar at the limit to 

 which weathering has reached. In Cornwall the kaolin mines, 

 which are probably the largest in the world, have reached a depth 

 of over 200 feet without the kaolin giving out, wliile at Zettlitz 

 in Bohemia a depth of over 400 feet has been reached with the 

 same result. The latter locality is one of thermal activity. In 

 these two instances tlie theory just mentioned seems tO' be ven,' 

 reasonable. There are many localities however where the kaolin 

 decreases with the depth, passing into the undecomposed feldspar,, 

 as is the case for example in J^orth Carolina,, where the fresh 

 feldspar is met at a depth of 60 to 120 feet. Still there are locali- 

 ties in the United States where the mineralizing vapors seem clearly 

 to have aided in the formation of kaolinite. Thus in ma,ny of the 

 mines at Cripplecreek in Colorado, kaolinite has been produced- 

 by the decomposition of the feldspar, and is considered by Penrose 

 to have been formed by other agencies than those of weathering, 

 for the reason that it shows no sign of decrease in quantity with 

 the depth, occurring as abundantly in the bottom of the deepest 

 mines as on the surface. The frequent association with it of the 

 unaltered sulfid minerals suggests that superficial alteration had 

 no part in the formation of the kaolinite, otherwise the sulfids 

 would have been oxidized to sulfate^. It is possible that fluorin 

 may have been the agent in the change, for it is abundant in many 

 of the Cripplecreek ore deposits. 



"VSTiatever the species of feldspar, or the process of decomposition, 

 the product is kaolinite, and, as previously observed, a mass of 

 kaolinite would be termed kaolin, or pure clay. Such a thing as 

 pure clay is however unknown, for one or more minerals are 

 always associated with the feldspar and remain in the kaolin as 

 impurities, but not necessarily injurious ones. Clay therefore is- 

 formed primarily by the decomposition of a feldspathic rock mass^ 



^ U. S. geol. sur. IQth ann. rep't pt 2, p. 131. 



