1918] Davis: The Radiolarian Cherts of the Franciscan, Group 265 



with a hand-lens, to make out any of the constituent grains of these 

 shales. In a few of them there are fine micaceous particles large 

 enough to be distinguished with slight difficulty by the aid of an ordi- 

 nary hand-lens. 



"When the red shales are examined with the microscope in thin sec- 

 tion they are seen to consist of a mass of extremely fine mineral par- 

 ticles, with which are intermingled grains of red iron oxide, in such 

 quantities as to make the slide almost opaque. 



These shales disintegrate very rapidly when subjected to weather- 

 ing action and, as a rule, they are exposed only in artificial cuts or in 

 sea cliffs. They disintegrate in contact with water and break down 

 rapidly into small irregular fragments with sharp corners and edges. 

 After breaking to a size where most of the particles are between one- 

 sixteenth and one-eighth inch in diameter, water seems to have no fur- 

 ther effect on the shale in the way of rapid disintegration. Even boil- 

 ing is then without noticeable effect. 



By treating the red shales with dilute acid it is possible to dissolve 

 out almost all of the iron oxide, leaving a residue having a pale green- 

 ish or cream colored tint ; rarely a very pale pink color remains. On 

 testing some of the residue after acid treatment in a qualitative way, 

 it is found to contain large amounts of silica, and alumina, a little iron 

 and managanese, together with small amounts of magnesium and 

 calcium. 



The more siliceous shales do not break down when the iron is dis- 

 solved out of them. The fragments of the shale retain their original 

 size and shape. Their hardness and compact texture art? about the 

 same as before, so that it would appear that the iron is simply a pig- 

 ment and has no cementing effect in the shale. Further treatment of 

 these fragments with a solution of caustic potash causes them to break 

 down into a slime. 



On breaking up the residue left after the acid treatment of the 

 shale and examining it under the microscope, it is found to lie a 

 crystalline aggregate of extremely fine grain. The mineral particles 

 show a subparallel orientation on rotating the stage between crossed 

 nicols. Though the aggregate never becomes totally dark, it shows 

 a brightening and darkening as the stage is revolved. This behavior 

 is shown by many ordinary shales, slates, and phyllites. 



While most of the minerals cannot be distinguished with certainty 

 on account of the extremely fine grain, there are occasional minute 

 flakes of a mineral which, from its fracture, limpid appearance, 



