Lawson 1 

 i'alacheJ 



The Berkeley Hills. 



425 



are very widely disseminated through them; limonite filling cracks 

 and cavities, and in streaks following the planes of lamination; 

 calcite abundant in veins and as the filling of vesicles; chalcedony 

 and rarely analcite as amygdules in the vesicular forms ; all these 

 secondary products form prominent features in the macroscopic 

 aspect of the rock. Under the microscope it is seen to be holo- 

 crystalline, but very fine grained, porphyritic crystals being very 

 exceptional. When there are such, they are plagioclase and augite 

 in imperfectly-formed crystals. The ground-mass is very feldspathic, 

 containing, besides the feldspar microlites, augite granules, and 

 magnetite in grains and rods. Here also secondary products are 

 abundant and prominent. Limonite is in streaks, spots, and con- 

 centrically-banded aggregates. Calcite replaces portions of the 

 original ground-mass, and fills cavities in concentric aggregates, 

 alternating with or surrounding limonite. Serpentine, ocherous 

 from included iron oxide, fills interstices of other constituents, and 

 replaces augite grains. Flow structure is well marked by paral- 

 lelism of the feldspars and by their tangential grouping about 

 phenocrysts and vesicles. 



The somewhat arbitrary division of the andesitic rocks into the 

 four facies above named and described, is justified on grounds of 

 convenience and simplicity. It is done, however, in full recog- 

 nition of the fact that in this, as in every natural group of volcanic 

 rocks, there exists a more or less uniform series of forms, merging 

 one into another by insensible gradations. The frequent occur- 

 rence of forms which can not be assigned with certainty to any of 

 our types is sufficient proof of the artificial character of our 

 grouping, which, notwithstanding, has its usefulness for purposes 

 of description. It is altogether probable that a more extended 

 study of the volcanic series over a wider area would not only 

 increase the number of varieties in the andesite group, but would 

 also reveal intermediate forms which would unite the andesites to 

 the basaltic group by a graded series of characters. The intimate 

 field relations of these two groups are such as to confirm the 

 suggestion that, though now regarded as specifically distinct, they 

 are essentially the different facies of the development of a single 

 magma. 



