THE DECOMPOSITION OF THE BOOKS. 79 



such as to preclude the idea that even this material contains any notable 

 quantity of kaolin. 



Secondary liquid inclusions. — Thc bcliavior of tlic particlcs of calcite which first 

 form in the feldspars is of some little importance, for these on leaching out 

 gi%'e rise to secondary liquid inclusions, as will be explained under slide 210.' 

 Such inclusions are met in all the decomposed andesites and appear to be 

 frequent also among the other rocks. If proper regard is paid to the con- 

 dition of the feldspars and to the shape of the inclusion, there is little 

 difSculty in discriminating between secondary and primitive liquid inclu- 

 sions, but a neglect of these precautions might readily lead to incorrect 

 diagnoses. 



Magnetite appears in certain cases to be converted into a yellowish- 

 white opaque substance, accompanied by polarizing grains, much resem- 

 bling calcite. The black border of hornblendes is sometimes wholly 

 removed in this manner, and the appearance of the rock considerably 

 modified. The phenomena suggest a conversion to a mixture of carbonate 

 of iron and limonite. 



Decomposition of rock-masses — Thc coursc takcu by the dccompositiou of masses 

 of rock depends largely on their physical character, and is sufficiently dis- 

 cussed in connection with the general description of each rock. Only porous 

 masses suffer decomposition uniformly throughout, and these are apt to retain 

 their coherence. Blocks of dense rocks are attacked from their surfaces and, 

 as in all processes involving solution or substitution, the corners and edges 

 yield more rapidly than the flat faces, so that the fresh kernel tends to assume 

 a spheroidal shape. The altered portion of the dense rocks frequently dis- 

 integrates. 



Condition of the quartz-porphyry. — The quartz-porphy ry throughout the District 

 and far beyond its limits is so much decomposed that not a single fresh horn- 

 blende has anywhere been found in it. P^xcept where increased by special 

 causes, such as propinquity to the Lode, the degree of alteration is also very 

 uniform. As it overlies very fresh granite and metamorphic diorite and is 

 overlain by fresh andesites, special causes must be sought to account for its 

 exceptional degeneration. None such have occurred to me except its phys- 



iPage 119. 



