358 University of California Publications in Geology [Vol. ll 



by Murray 122 to be the results of chemical wastes thrown into the 

 River Clyde. 



As far as grains of manganese oxide are concerned, they occur in 

 all terrigenous muds, as shown by analyses. Away from the deposits 

 of manganese ore, the Franciscan cherts and shales contain only small 

 amounts of manganese oxide, in spite of the fact that it is ' always 

 present in them. The content of manganese oxide in the Red Clay, 

 1.21 per cent, is much higher than the content of manganese in the 

 ordinary chert or shale of the Franciscan. The percentages of man- 

 ganese oxide in radiolarian oozes are also higher than those of the 

 Franciscan rocks. 



Moreover, as far as the Franciscan cherts and shales are con- 

 cerned, nothing has ever been found in them which resembles in the 

 slightest degree the potato-shaped nodules of manganese so often 

 brought up from abyssal depths by deep sea dredgings. 



Another fact lias recently been brought to light by Mr. G. C. 

 Gester.* He has found evidence, in certain manganese deposits in 

 the Franciscan cherts, that the original mineral was not manganese 

 oxide, but manganese carbonate. Near the surface, the carbonate is 

 changed into the oxide. The carbonate of manganese appears to have 

 been originally deposited in lenses parallel to the bedding of the 

 cherts. This discovery is opposed to the interpretation of the cherts 

 as abyssal radiolarian oozes, for the reason that the manganese in the 

 deep sea appears to be always in the form of manganese oxide and 

 not in the form of the carbonate. 



Bed Color. — Certain facts indicate that the original color of a 

 large part of the Franciscan cherts and shales were red — or at least 

 that the iron was deposited in the ferric state. The only alteration 

 of color which has been seen in the cherts, aside from changes due to 

 intrusive action, is a change from red to green, due to leaching of 

 waters circulating along fissures. The red color in the cherts bears 

 no relation to the surface, and it does not seem possible that a general 

 oxidation of the formation could have occurred. The conclusion that 

 the color of the cherts and shales was originally red agrees well with 

 the idea that they represent the radiolarian oozes and the red clay. 

 The extreme fineness of grain of the red shales is also in accordance 

 with the idea that they may represent abyssal red clay. 



Chemical Composition of Shale. — If one compares the silica con- 



122 " Challenger ' ' Report. 



* Personal communication, April, 1917. 



