852 A TREATISE ON METAMORPHISM. 



from the parts of the formation once above the plane of denudation. The 

 cherty formations of the Lake Superior region are usually steeply inclined, 

 and therefore descending water rapidly and easily segregates silica on a 

 large scale from material widely disseminated. Where such substitution 

 occurs throughout the outcrop of a tilted formation it does not at all follow 

 that it has taken place throughout the formation. Indeed, in the majority 

 of the districts of the Lake Superior region it is probable that the segrega- 

 tion of chert extends to only a very limited depth. Where the chert is 

 thus segregated by descending waters the source of the material may still 

 be mainlv chert of organic origin originally disseminated through the 

 carbonate. 



Where there are rather extensive formations of chert in a horizontal 

 position, or nearly so, interstratified with beds of limestone or shale, the 

 transportation of silica from material once above the plane of denudation by 

 descending waters is not so likely to have been important. This is espe- 

 cially true of those cherts which are interstratified. with shales, and therefore 

 are protected from the surface waters by impervious formations. Hence 

 such formations are more likely to be in large part the direct products of 

 organic precipitation. While bunchy deposits of chert having no great 

 lateral extent may often be produced by chemical precipitation, it is certain 

 that such deposits may also be produced b}^ direct organic precipitation. 



This is illustrated by the chert deposits of the Franciscan series of 

 California. These are regularly bedded deposits, locally of considerable 

 thickness. They are lenticular and have an extremely narrow lateral 

 extent. Often a deposit having considerable thickness disappears within a 

 short distance. If a bunchy character be regarded as evidence of chemical 

 origin it would be concluded that the Franciscan cherts are chemical 

 deoosits, and this was Lawson's conclusion." Yet Lawson states that these 

 cherts contain very numerous radiolaria; but this fact does not prevent 

 him from suggesting that they are mainly due to local precipitation, in the 

 bed of the ocean, of silica from silica-bearing springs. Lawson explains the 

 abundant radiolaria as a coincidence. He says, in these deposits "radio- 

 laria remains became embedded as they dropped to the bottom.'"' It 



« Lawson, A. C, Sketch of the geology of the San Francisco Peninsula: Fifteenth Ann. Rept. 

 V. S. Geol. Survey, 1895, pp. 42(M26. 

 b Lawson, cit., p. 426. 



