Law son -j 

 I'alacheJ 



The Berkeley Hills, 



427 



II. Grizzly Peak andesite, porphyritic facies, southeast of Grizzly Peak. 

 Analyst, Palaehe. 



III. Grizzly Peak andesite, holocrystalline facies, head of Telegraph 

 Canon. Analyst, Palaehe. 



IV. Summit of Grizzly Peak. Analyst, Palaehe. 



V. Aphanitie laminated andesite, Pie Knob. Analyst, Palaehe. 

 VI. Aphanitie laminated andesite, Pie Knob. Analyst, Dickerman. 



In the rock whose composition is given in VI, there is about 6. 1 2 

 per cent of free ferric oxide which as limonite occupies largely the 

 open spaces in the rock, and probably has a source foreign to the 

 rock itself. In the analyses given, this 6.12 per cent ferric oxide 

 has been deducted and due correction made. The rock analyzed 

 is otherwise fresh. 



THE BASALTS. 



The basaltic lavas are scarcely subordinate to the andesitic in 

 this field. They occur in both Upper and Lower Berkeleyan and 

 in the Campan as important contributions to the accumulations 

 which make up those series. They are found also to a small 

 extent in dyke form. Although they occur, as the andesites do, at 

 many different horizons, and have in the aggregate about the same 

 volume and lateral extent, they are much more uniform in character 

 and exhibit but slight variations in structure and mineralogical 

 composition. 



Constituent Minerals. — Mineralogically the basalts consist essen- 

 tially of plagioclase augite and olivine, with accessory apatite and 

 magnetite. Glassy matter is present in some lavas. Serpentine 

 and iddingsite are the most important secondary products. 



The feldspar is in two generations. The porphyritic crystals 

 are sometimes sharply idiomorphic, more generally with indistinct 

 and irregular outline or rounded and embayed by magmatic corro- 

 sion. Twins formed on the albite plan are numerous, and Carlsbad 

 and pericline twins are infrequently seen. Extinction angles meas- 

 ured on albite twins which extinguished symmetrically, were as large 

 as 36^ degrees, indicating that the feldspar has the composition of 

 bytownite or anorthite. Zonal forms are common, but less so than 

 in the feldspars of the andesites. The smaller phenocrysts of feld- 

 spar are usually quite free from inclusions; the larger ones, which 



