﻿CLAYS FROM LUZON. 



415 



the characteristic breaking down in the formation of residuary clay de- 

 posits is as follows : 



rick 



3*1 



Fig. 1. 



One of the best examples known to me where such decomposition is 

 now going on is in northern Tiizal along Maeoburobod Creek. This 

 stream empties into Poray Creek, San Mateo River, about 10 kilometers 

 above Montalban. Its waters are extremely acid from the decomposition 

 of sulphides. Along its banks is an outcrop of vein material carrying 

 quartz which is characteristically weathered into clay and which shows 

 the whole process taking place in the formation of primary clay deposits. 

 The quartz is unaffected by the action of these waters and remains as 

 grit in the final product. Wherever we have grit in a clay, we can almost 

 invariably attribute it to unweathered particles of quartz which were 

 present in the parent rock. An emulsion of the above clay was filtered 

 through a sixty-mesh sieve. This filtrate, when thoroughly air-dried 

 represents the result of the decomposition in this vein and the product 



is as follows : 









Constituent. 





Per cent 



Silica 







59.00 



Alumina including 



titanium 



oxide 



10.79 



Ferric oxide 







7.87 



Ferrous oxide 







0.42 



Lime 







0.40 



Magnesia 







0.00 



Alkalies 







3. S9 



Loss on ignition 







7.4(5 



Water below 110° 







3.18 



Sulphur 







4.70 





101.80 



Less oxygen 







1.70 



Total 



100.01 



The formation of a great many of the clay deposits in various parts 

 of the world is attributed to causes almost identical with the above. 

 Bacon, in connection with his investigation of the acid waters of "The 



This Journal. Sec. A. dm. Sri. (1907). 2, 120. 



