﻿414 cox. 



of the pure, parent rock seldom takes place unassociated with that "of 

 other minerals, foreign substances in small amount being always mechan- 

 ically mixed with it ; however, because of its almost universal presence. 

 it is often considered the basis of all clay. Kaolinite has the composi- 



/OH 

 tion: Si,0 9 Al,H 4 ; 3 or perhaps better Al — Si0 4 EH 3 which indicates its 



\Si0 4 EAl 

 relation to the parent mineral 4 : it contains alumina 39.45. silica 46.65, 

 and water 13.9 per cent, respectively. 



Other clays result not only from the decomposition of feldspar, but 

 from the breaking down of other associated crystalline silicates to which 

 they are related somewhat as kaolin is to the feldspars, and they also 

 contain foreign matter which is mixed with them. Usually these mix- 

 tures are so complicated that it is almost impossible to identify them. 

 Just as the clays range from pure kaolin to impure varieties, so in the 

 end, because of a greater proportion of certain of the minerals in the 

 parent rock, the products of decomposition may lose all the characteristic- 

 physical features of a clay and thus pass out of this class of bodies 

 entirely. 



Clay deposits are usually designated as residuary or sedimentary. As 

 the names imply, these originate from their method of formation, accord- 

 ingly as this is due to the decomposition of rocks in place or to the 

 resulting decomposition products being transported by the streams and 

 deposited in regular sedimentary beds elsewhere. Kaolin is termed a 

 primary or residuary clay because it is always in the condition of original 

 deposit. This is true of that from Laguna : that is, it is in place but 

 thoroughly leached out. The formation of all clay begins in the same 

 way. by the decay and disintegration of the rock mass. In this Archi- 

 pelago, where we have no frost during the year, the principal active 

 agents accomplishing this change are water and the sun. Dana 5 says, 

 "Feldspar may be altered through the action of waters rendered acid by 

 the decomposition of sulphides.*" Such waters filter into the rock, causing 

 partial disintegration, then the drying action of the sun opens cracks for 

 more water to enter and further to attack and break down the mineral 

 grains in the rocks. By means of such infiltrating waters the feldspar 

 of the feldspathic rocks acted on, first loses its lime, which is soon de- 

 posited as gypsum (calcium sulphate), next its alkalies as sulphates, and 

 the change finally ends in kaolin or some other silicate of aluminium. 

 In the less resistant portion of the vein the weathering will penetrate to 

 a much greater depth than in the harder parts. An ideal section showing 



* Grothj P.: Tabellarische trbersicht der Mineralien. Braunschweig (1898), 137. 



4 Clarke.. F. W.: U. 8. G. 8. Bull. (1905), 125, 28: McNeil, H. C: Journ. Am. 

 (Them. Soc. (1900), 28, .592. 



5 System of Mineralogy, New York (1900). 320. 



