ALTERATION OF THE LATER ANDESITE. 245 



and sulphur, and there has resulted partial hydration, oxidation, carbonation, and 

 sulphuration. The minerals developed, as shown by the microscopic description, 

 are serpentine, siderite, calcite, kaolin, quartz, hematite, and pyrite. The consid- 

 erable degree of oxidation of the iron, as compared with C and E, is shown by 

 the analysis. There is no evidence, however, that this incipient decomposition has 

 been attended by any change in the relative amount of the rock constituents; it 

 was rather a rearrangement of the materials into new minerals that were more 

 stable under the new conditions. 



PRINCIPLES OF STUDYING ALTERATIONS OF LATER ANDESITES. 



No attempt has been made to follow the different stages of the alteration of 

 the later andesite by analysis, as in the case of the earlier andesite, although these 

 stages have been minutel} 7 studied under the microscope. Therefore, while the 

 first two analyses (p. 241) are of the freshest rocks obtainable, the last two, 3 and 

 4, are of entirely decomposed rocks. In 3 and 4 not only has the original mineral 

 composition, as shown by microscopic examination, been completely obliterated, 

 but in the process there has been an important change in the chemical composi- 

 tion of the rock as a whole. This is well illustrated by the diagrams forming 

 figs. 75 and 76. 



It will be noted that in all four analyses the amount of alumina remains 

 practically constant. This oxide is perhaps the most refractory among rock 

 constituents, and computations in regard to loss or gain during rock alterations are 

 often based on the assumption that alumina remains unaltered. That it probably 

 does not exactly do this, under intense action, is shown by the study of the earlier 

 andesite analyses, where the percentages of alumina in the bulk analyses decrease. 

 The constancy of the alumina in the four later andesite analyses under consid- 

 eration, however, is taken to indicate that the alumina has not been noticeably 

 attacked by the alteration, and therefore that the comparison of the percentages 

 of the other constituents affords an approximately correct idea of the loss and gain. 





