Fairbanks. | 



Geology of Point Sal. 



2 I 



being perfectly disintegrated and ready to fall in pieces on expo- 

 sure. Where the canons have cut across, it is somewhat harder, 

 but has even there the appearance of considerable alteration. 

 Feldspar and augite, the latter in fresh, lustrous crystals, appear in 

 the hand specimen, in addition to analcite and other alteration prod- 

 ucts. It is, however, in the large outcrop in the cliffs that the 

 rock is best exposed in all its interesting facies. Although the 

 ■cliffs are nearly 200 feet high, the rock at the bottom where the 

 waves are eating into it seems fully as decomposed and disinte- 

 grated as that near the surface. 



While there are portions of the mass which are solid and appar- 

 ently fresh, as shown by the large boulders at the foot of the cliffs, 

 much of that even at the shore line gives a dull thud when struck 

 and crumbles with a slight blow. The variation in structure and gen- 

 eral appearance in this outcrop is remarkable. An aphanitic gray 

 specimen from the edge of the mass has the appearance of being 

 sporadically amygdaloidal. Toward the northern side there are 

 areas of considerable size in which the rock very closely resembles 

 the Cuyamas eruptive. It is a coarse, light-colored rock, with lath 

 feldspars, dark, lustrous augites, and abundant analcite in large 

 grains. This light-colored rock is often more coherent and seems 

 to shade off into the darker and more friable portions insensibly in 

 places, in others to project into it in dike-like tongues with quite a 

 noticeable difference. The contrast is sufficiently strong in places 

 to make it appear as if one were intrusive in the other. In most 

 places the distinction between the more coherent and that less so, 

 and between the coarse facies and the finer grained, is not sharp. 

 A large area in the center of the outcrop has a peculiar character. 

 This consists in the presence of nodular kernels of fresh, hard rock 

 imbedded in a decomposed matrix of apparently similar composition. 

 A close examination showed that these nodules, averaging perhaps 

 a half inch in diameter, were as a rule each composed of a poikilitic 

 augite crystal filled with minute lath feldspars. This was made 

 apparent by breaking one, when the fracture would be seen to be a 

 cleavage plane of the augite. No sharp demarkation appears to 

 exist between this rock with the poikilitic structure and that with 

 the panidiomorphic. So abundant is the feldspar in the augite that 



