318 University of California Publications in Geology [Vol. 11 



admixture of minute chips of quartz, and flakes of mica which appear 

 to be of detrital origin. These grains are from .03 millimeter to .065 

 millimeter in diameter. 



The microscopic characters of these cherts are much like those of 

 the radiolarian cherts of the Franciscan formation. One point of dif- 

 ference is to be noted. In the Codden Hill cherts there are minute 

 casts of rhombohedral crystals. These are probably left after the 

 solution of calcite or dolomite. In some instances these casts have 

 later been filled with silica. 



Beside the radiolaria, remains of brachiopods, corals, crinoids, and 

 goniatites, have been found occasionally in these cherts, or as casts 

 in beds of siliceous shale. 



Hinde and Fox regard these deposits as similar to the present 

 radiolarian oozes and believe that the Codden Hill beds were laid down 

 in the deep water of an open sea. 



GOWER 



Dixon and Vaughan 60 have described Carboniferous radiolarian 

 cherts from Grower. Here fifty feet of radiolarian cherts lie over soft 

 argillaceous limestones and shales. There is no evidence of uncon- 

 formity below the cherts, though there is an abrupt change in the 

 nature of the rock. While the upper contact is not well exposed, the 

 cherts appear to pass upward into shales and sandstones. 



The radiolarian cherts are interbedded with thin layers of shale. 

 The cherts and shales are well laminated. The shales are barren of 

 fossils except for a few lamellibranchs and indeterminate plant 

 remains. 



Under the microscope the cherts are found to contain abundant 

 radiolaria and a few sponge spicules. The lamination is due to dif- 

 ference in color and grain of the rock, and in the proportions of 

 organic and inorganic matter. The laminae are lenticular, many 

 being sharply so. The inorganic matter referred to is said to be fine 

 quartz silt. 



These cherts are regarded by Dixon as constituting a "lagoon 

 phase," which is defined as a group of rocks whose characters show 

 that they were deposited in a coastal area of wide extent but ex- 

 tremely shallow — a wide coastal shelf having a long extent parallel 

 to the shore. Due to the extreme shallowness of the water it is cut 



eo The Carboniferous Succession in Gower, Quar. Jour. Geol. Soc, vol. 67, 

 p. 477, 1911. 



