ANDE8ITE OF MOUNT DOANE. 315 



were intrusive bodies they had reached very close to the surface of the 

 breccias. 



Two of the most conspicuous bodies of massive andesite form the 

 conical summits of Mount Doane and Mount Stevenson, and constitute the 

 upper 500 to 900 feet of these mountains. The mass at Mount Doane con- 

 sists of light-gray compact hornblende-mica-andesite, filled with minute 

 crystals of feldspar, hornblende, and biotite, with a few larger phenocrysts 

 (1575, 1576). Near the bottom contact it is dark colored and weathers 

 into small rounded masses. Higher up in the body it is columnar, passing 

 into platy parting, in more or less vertical slabs. 



The groundmass of the rock from near the bottom contact is micro- 

 crystalline and microlitic, and is gray in thin section. The phenocrysts 

 are greenish-brown hornblende and brown biotite, both without black 

 borders or any noticeable inclusions of magnetite. Magnetite also forms 

 small phenocrysts, the size of the small ones of hornblende and biotite. It 

 is abundant as minute crystals in the groundmass. The labradorite is cen- 

 trally and also zonally altered to a brown substance, probably kaolin. In 

 the rock from the summit of the mountain the groundmass is slightly more 

 coarsely crystallized, and the structure is almost microgranular, the grains 

 being about 0.015 mm. in diameter. The phenocrysts are the same as in 

 the finer-grained varieties, but the hornblendes and biotites are filled with 

 minute grains of magnetite, some of the smaller hornblendes being almost 

 completely replaced by it, and also by some pyroxene. Magnetite forms 

 small phenocrysts, and there are a few of augite. The labradorite is almost 

 fresh. A cross section of the whole body is shown in the almost vertical 

 cliff at the northern face of the mountain. The massive andesite is seen to 

 be resting on almost horizontal layers of light-colored breccia, the plane of 

 the bottom contact being curved, and rising several hundred feet at the east 

 and west ends of the section. The andesite body appears to have been a 

 surficial flow which filled an ancient drainage channel. 



The same appears to be true of the massive hornblende-mica-andesite 

 forming the top of Mount Stevenson (1577 to 1581). This rock is light 

 gray, with more prominent hornblende than in the mass of Mount Doane, 

 and contains segregations of hornblende and feldspar, with a little biotite. 

 The groundmass of the light-gray rock is holocrystalline, with a mixture of 

 feldspar prisms and grains, besides magnetite and pyroxene grains. The 



