GENERAL GEOLOGY 43 



in large scales changing to chlorite ; the hornblendes in distinct por- 

 phyritic crystals. 



Altered biotite-andesite (168). A pearl grey felsitic base, with 

 opaque white porphyritic feldspars and distant black rods of biotite. 

 With the microscope the whole is seen to be much altered, the plagio- 

 clase kaolinized, and the biotite changed to bright green chlorite. 

 Many cavities are filled with secondary quartz. 



Hornblende-andesite (206). This is a dark green compact rock 

 with large brown porphyritic crystals of hornblende. It has a dis- 

 tinction of color as if it were brecciated. In one part the large por- 

 phyritic feldspars have many inclusions, the hornblendes have black 

 resorption rims, and the biotite is changed to chlorite. In the other, 

 all these minerals are brecciated but lie in a cryptocrystalline ground 

 which is common to the two kinds. 



Diabase (170). A clear grey harsh fine-grained rock with quite 

 large cavities full of chlorite (delessite). It has the perfect ophitic 

 structure, and the augite is much altered to chlorite. It contains inclu- 

 sions of quartz with reaction rims of augite. 



Olivine-diabase (171). A dark grey fine-grained harsh rock. The 

 structure is almost ophitic, but the many short laths of feldspar and 

 small crystals of augite are wholly idiomorphic and so approximated 

 that an even thin seam of the black-dusted glassy ground intervenes. 



Aplite (169). A fine-grained flesh-colored granitoid rock with 

 miarolitic cavities containing orthoclase and complex quartz crystals. 

 This occurs very abundantly in large blocks among the beach cobbles. 

 The whole field is taken up by the most beautiful micropegmatitic 

 structure, which surrounds and radiates from albite phenocrysts. The 

 structure is well marked in ordinary light, since the whole is evenly 

 kaolinized. 



PORT CLARENCE 



At Port Clarence, on the Alaska coast south of Ber- 

 ing Strait, we sailed past a magnificent flat-topped ter- 

 race, about 500 feet high, on the north, rounded Point 

 Spencer, and cast anchor inside the long gravel spit of 

 which this is the apex. We then went east across the bay 

 in a launch, and landed at the mouth of a stream to take 

 water. In fig. 1 1 the mouth of the stream is shown about 

 two-fifths of the distance from the right of the picture. 



