1918] 



Davis: The Franciscan Sandstone 



33 



This conclusion is in agreement with ideas advanced by Mackie r '- 

 as a result of his studies of sedimentary rocks, recent deposits of vari- 

 ous kinds, and the mineral composition of stream transported sands. 

 Mackie believes that high percentages of fresh feldspar indicate either 

 a very cold, or else a warm and dry climate. He states that in the 

 absence of ice action, one should conclude that the climate was warm 

 and dry. No evidence is known which would suggest ice action during 

 the Franciscan, though the possibility of a cold climate is discussed 

 later. 



Conditions of Deposition 

 SUGGESTION OF CONTINENTAL DEPOSITION 



While the considerations just set out indicate the probability that 

 the Franciscan sandstone was laid down under an arid climate, the 

 petrographic nature of the rock does not furnish much evidence as to 

 the conditions of deposition. 



The absence of fossils from the greater part of the sandstone 

 strongly suggests that the sandstone was deposited subaerialby. 



The presence of much feldspar and the angularity of the grains 

 indicate little abrasion and show that the material was not carried for 

 long distances in streams. By this the possibility of continental depo- 

 sition is again suggested. 



POSSIBILITY OF WIND ACTION 



It seems certain that the deposit was not due to wind action, nor 

 was it produced under conditions in which wind played an important 

 part. The grains are not rounded as wind blown grains should be, 

 nor are they frosted like wind blown sands. The sandstone never 

 shows dune stratification, and exhibits no faceted pebbles. 



FLUVIATILE DEPOSITION ON PIEDMONT SLOPES 



The sands which now compose the Franciscan sandstone may have 

 been deposited by rivers on a piedmont slope which lay to the east of 

 a young mountain range, produced at the time of the intrusion of 

 the Coast Range granite. Such a mountain range would act as a 

 barrier against moisture laden winds coming in from the Pacific 



52 Mackie, W., Trail. Edin. Geol. Soc, vol. 7, p. 443, 1898. 



