GEOLOGY OF SAN FRANCISCO PENINSULA 



67 



formed, and .t another siliceous. Tire loeal bands »£ J''l'';='- ^ 

 L fee perhaps in lateral extent, wholly surrounded by hme- 

 one and sharply d.fferentiated from rt could not m all proba- 

 b li V have been formed through the action of s.hceous spr.ngs^ 

 The large lens-shaped bodies of mass.ve jasper ar also 

 .bruptlv marked off from the inclosing sandstone o"- e. Heach 

 lens were due to the action of one or more sprmgs the ct,rrents 

 Tst of necessity have been weal, near the edges of the depos 

 and could not possibly be conceived as "sufficient to deflect 

 "diment-laden ounter-currents." If sedimentation were going 

 o r lund the springs it is impossible that the silica cot. have 

 been precipitated without more or less commingling with the 

 sand The presence of similar radiolarian ,asper m the hn e- 

 one IS suggestive of the view that to whatever cause the 

 Tnticular fofm of the latter is due the same cause conditioned 

 the similar outline of the former. 



Turner, says of the limestones in 'he KnoxviUe a Moun 

 Diablo and of the older limestones in the Sierras that each 

 "Tcare'ous layer is rather -eries of lerUicular od.s tha„_a 



rras::u:r;thr:h:-l.mi:;:neTodies are hundreds 

 of feet in diameter." The belt of foraminiferal limestone 

 described by Professor Lawson is equally bunchy, reaching a 

 V "consid table thickness in places, as on the northern slope 

 o Bl ck Mountain, and then contracting to very .m.ted propo - 



ons. It thus appears that as far as the shape of t^e deposits 

 concerned the spring theory of ongm is equally applicable 



"^^Tirrements in the report concerning the condition of the 

 formation of the jaspers do not exactly -"i- ^ F- inst 

 on oaire 426, Professor Lawson says. It the springs 

 fong the cu rents engendered might in some places have bee 

 sufficfent to deflect sediment-laden counter-currents, and his way 

 s fve to explain the general absence of clastic material in the 

 chert." On page 466 he says : " At different more or less pro- 



'Bull. Geol. Soc. Am., Vol. Ill, P- 394- 



