Fairbanks.] 



Geology of Point Sal. 



15 



distinguishable mollusc is Pectcn peckhami, Gabb. The change from 

 silicious to calcareous deposits through a series of beds in which 

 waterworn fragments are absent, and microscopic organisms abun- 

 dant, is due, doubtless, to a change in the conditions of ocean life. 



Gypsiferons Clays. — The beds which come under this division, 

 though possessing a varied character, differ markedly from the 

 bituminous shales. They consist of gypsum-bearing clays promi- 

 nently developed along the southern slope of the main ridge, where 

 they reach a thickness of at least 1,800 feet. Immediately below 

 them lies the most important of the three strata of volcanic ash. 

 Below the ash there is a thickness of nearly 2,000 feet of soft arena- 

 ceous sandstone, shale, and conglomerate. The greatest thickness 

 ■of these beds is exposed in the first canon east of the Chute. The 

 conglomerates at the bottom alternate with beds of very soft sand- 

 stone and consist of pebbles of sandstone and jasper identical in 

 character with the rocks of the Coast Ranges, which the author be- 

 lieves are of pre-Cretaceous age. These pebbles predominate, but 

 there are also present in abundance others of a basic eruptive, 

 probably serpentine, but so oxidized that they appear as round red 

 bunches. They give a characteristic color to the basal portion of 

 the series. Pebbles of gabbro are also present in places. The con- 

 glomerates rest on the diabase and gabbro, dipping north at an angle 

 of 50 to 6o°. Faulting was not noticed in the cross section 

 east of the Chute, but along the coast from Point Lospie to the 

 teschenite cliff, near the Old Landing, the beds are fauited and 

 folded in short waves. The clays proper extend northwest to the 

 -spot where the landing was located. Here they have been intruded 

 by the teschenite dikes. Owing to the friable nature of the latter 

 rock and the very steep slope, slides are constantly occuring in wet 

 weather. These lower Miocene beds are exposed for a mile and a 

 half along the coast north of Point Sal, being overlaid by the Pleis- 

 tocene. They have much the same character as on the south side 

 of the ridge. They rest against the spheroidal basalt at a very 

 steep angle, and have been so greatly compressed and folded that 

 the thickness could not be estimated. Near the base the sand- 

 stone is filled with concretions three to six inches in diameter. 

 They stand out so thickly on the bedding surface of the upturned 



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