1918] Davis: The Radiolarian Cherts of the Franciscan Group 271 



large grains, in which are rounded masses of iron oxide considerably 

 larger than the grains of iron oxide in the unaltered rock. All traces 

 of radiolaria have disappeared in consequence of recrystallization of 

 the rock. The yellow or orange contact phase is very similar in gen- 

 eral appearance. In a section of one of these cherts the coloring mat- 

 ter is bunched together in little balls which, under high powers, are 

 seen to be interlacing fibers of some yellow mineral. The matrix is 

 chalcedonic silica, shown by the fibrous extinction of the grains. 



Development op Quabtzites 



Sometimes rocks resembling quartzites result from the alteration 

 of the cherts. In the field these rocks are massive and appear in promi- 

 nent outcrops, showing little trace of the original bedding. They may 

 be deep red, orange, yellow, brown, or sometimes black. They are 

 vitreous rocks which, on fresh fractures, look almost homogeneous. 

 Upon careful observation, however, faint outlines of individual crys- 

 tals may be seen. On weathered surfaces these rocks show, in places, 

 a peculiar glazed appearance as though they had been fused. The 

 cause of this is unknown. In thin sections, the rock is seen to be made 

 up of a mass of interlocking quartz grains, which are fairly transpar- 

 ent but contain a number of very minute, transparent inclusions, too 

 small to be determined. Often the iron oxide is confined to the spaces 

 between grains of quartz. In some sections the mineral which colors 

 the rock is blue-black with the characteristic luster of magnetite. On 

 testing the powder it is found to be magnetic. There are also some 

 grains of a dark red mineral, probably hematite. 



In other sections of quartzite the iron is present in the form of 

 yellow oxide which in part is fibrous. It shows sharp, clear boundaries 

 against the quartz, and does not appear to be due to alteration of 

 other forms of iron oxide, but to have been produced at the time the 

 rock recrystallized. 



Quabtz Veins 



In the ordinary, unaltered chert the quartz veinlets run normal to 

 the bedding. Near contacts with igneous rocks veinlets are seen which 

 may run parallel to the bedding of the chert instead of across it. 

 Such veins are irregular in thickness, often being lenticular, whereas 

 the cross veins in ordinary chert are fairly uniform in thickness. 



In the massive chert peculiar, curving, lenslike veinlets of quartz 

 may be seen, arranged roughly in concentric groups. Plate 31b repre- 



