GEOLOGY OF SAN FRANCISCO PENINSULA 65 



cases thev are true jaspers and have been so designated in some 

 o the earner deser.ptions of them." After discuss.ng the.r 

 variability he again says : " In view of this variation nt petro- 

 : phic character, it has been deemed best to refer to these 

 focks by the old and familiar name of 'chert.- It seems to me 

 that, on the contrary, the designation "chert" is^ not only not 

 as appropriate for rocks of this character but it does away wrth 

 ..an old ind familiar name" for no suflfieient reason. T'- desig- 

 nation "jasper" was used by Blake, Newberry, and Wh.tney. 

 According to Geikie, "Chert is a name applied to .mpure cal- 

 careous varieties of Hint in layers and nodules which are found 

 among the Pateozoic and later formations, especially but not 

 exclusively in limestones." 



The siliceous bands in the foraminiferal limestone which a.e 

 referred to by Professor Lawson as "veins" I do not consider 

 such in any true sense of the word ; they are more properly 

 cherts or phthanites according to the original use of the terms, 

 but in regard to the great body of siliceous rocks occurring inde- 

 pendently there seems to be no use in making a change in terms 

 The nature of the so-called " veins" inthelimestone seems not 

 to have been clearly understood. On exposed surfaces of many 

 feet in extent these siliceous bands stand out with grea distinct- 

 ness Some of them are as even and regular as the limestone 

 strata while others are discontinuous and more uneven. Ihey 

 sometimes blend into the limestone but more commonly are 

 quite sharply distinguished. These bands of chert are contem- 

 'aneous'd'eposits, being always conformable to the stratifica- 

 Uon of the limestone, and differing most markedly from the veins 

 of secondary origin. Thin slides prepared from a number of 

 specimens show them to be thickly filled with organic remains 

 of radiolarian character. . 



After a description of the jaspers several theories o their 

 origin are considered. The theories are as follows : (l) siliceous 

 springs in the bottom of the ocean, similar to those well known 

 in vofcanic regions; (.) radiolarian and other siliceous remains 

 which mav have become entirely disssolved in sea water, (3) 



