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University of California. 



[Vol. i. 



characterized by a more or less distinct shear structure, parallel with 

 the original bedding planes. This is often not very pronounced, 

 and appears to be unaccompanied by any chemical alteration except 

 near the contact with intrusive rocks. 



Owing to the discontinuous nature of the rock exposures over 

 the interior of the island, it was not possible to select any character- 

 istic bed upon which to construct a stratigraphic series. The thick 

 beds of Quarry Point are, however, regarded as forming the base 

 of the series exposed, being succeeded by the thinner beds and 

 conglomerate bands, the last named being in the upper portion. It 

 is believed that 2,000 feet is a very conservative estimate for the 

 thickness of the sandstone beds exposed upon Angel Island; the 

 true figure is probably in excess of this. There has been consider- 

 able disturbance in the strata, as would be expected from their pre- 

 Tertiary age and from the extensive scale upon which local igneous 

 intrusion has taken place. Consequently the structure is somewhat 

 obscure. In a general way, however, the beds may be regarded as 

 forming a deep syncline with its axis running about northwest and 

 southeast; the mapping, moreover, seems to indicate the presence 

 of a subordinate buckle, or low anticlinal ridge, occupying the bottom 

 of the trough. This syncline is tilted up so as to slope to the north- 

 west, and the semicircular amphitheatre rising above Hospital Cove 

 is a feature conditioned by structure and revealed by erosion. That 

 the syncline is not a simple fold is readily seen by reference to the 

 dips and strikes plotted upon the map, which plainly show the exist- 

 ence of minor corrugations, but the recognition of its presence as 

 a broad structural feature is the simplest construction to be put 

 upon a stratigraphy whose minuter structure is hopelessly intricate. 



The whole series of sandstone beds bears evidence of a rapid 

 and variable accumulation, which probably accounts for the absence 

 of fossils, no trace of animal remains having been found in them. 



The Radiolarian CJicrt. — Interbedded with the sandstone, and 

 occurring in small isolated patches within it, are numerous masses 

 of the ordinary jaspery rocks usually associated with the San Fran- 

 cisco sandstone. Although in many localities the name "bedded 

 jaspers" may be applied to these rocks with perfect propriety, yet 

 the investigation of their representatives upon Angel Island, appears 



