DISTRIBUTION UNITED STATES 619 



tains inclusions of red and green radiolarian cherts, through which it 

 breaks. 



Fairbanks^^ has described a basalt from Point Sal, near San Francisco, 

 which consists of elongated ropy masses packed over each other in all 

 conceivable positions. A rude parallelism is apparent at one exposure, 

 and some of the masses approximate the spherical form^ with diameters 

 ranging from a few inches to 3 feet. In many places also this structure 

 is not sharply differentiated from the structureless basalt. The outer 

 portions of the masses are more compact and have smaller amygdules 

 than the inner, and hence they often weather out hollow. 



Lawson^^ regards all the variolitic spheroidal basalts and diabases of 

 the region around San Francisco Bay (plate 18, figures 1 and 2) as in- 

 trusives cutting the Franciscan series. These rocks occur at many places 

 within the area of the four quadrangles included in the San Francisco 

 Folio. 



"They are of irregular shape, they include no clean-cut dikes or intrusive 

 sills, and their exposed contacts with the rocks they intrude are generally 

 irregular and jagged. Fragments of the incasing rock, especially of the 

 radiolarian cherts, are abundant at the contacts, where the chert is usually 

 baked to a bright vermilion-red and its structure is in some places also greatly 

 changed. Some inclusions show evidence of partial resorption. The sphe- 

 roidal structure of these intrusive rocks is clearly revealed only on sea cliffs, 

 as at Hunter Point and Point Bonita, but may also be detected in numerous 

 road cuttings and on natural exposures on hillsides. On sea-cliff exposures 

 the rock presents the appearance of an irregular pile of filled sacks, each sack 

 having its rotundity deformed by contact with its neighbor. Each of these 

 sack-like or ellipsoidal masses measures about 3 feet in its longest and about 

 1 foot in its shortest diameter. The rock between the ellipsoids is usually 

 more decomposed than that elsewhere and so weathers out easily under the 

 action of the waves, leaving the more resistant ellipsoids prominent. Some 

 of the ellipsoids are vesicular, others are variolitic, and still others are both. 

 The cause of this peculiar structure and the mode of its development are 

 not yet understood." 



Alaskd. — Grant and Higgins^^ have described the ellipsoidal green- 

 stones of the Orca group (Mesozoic) at Prince William Sound, Alaska 

 (plate 19, figures 1 and 2), in which the spheroidal masses range from a 

 few inches to 10 feet in diameter. In many places these bodies make up 



81 H. W. Fairbanks : The geology of Point Sal. Bull. Dent, of Geol., Univ. of Cal., 

 vol. ii, 1896, pp. 1-92. 



82 A. C. Lawson : San Francisco Folio, U. S. Geol. Survey. I am greatly indebted to 

 the Director of the Survey for permission to consult the proofs of this folio in advance 

 of publication. 



^ U. S, Grant and D. F. Higgins : Reconnaissance of the geology and mineral re- 

 sources of Prince William Sound, Alaska. Bull. U. S. Geol. Survey, No. 443, 1910, pp. 

 21, 26, 51, 52. 



