GEOLOGIC RECORD OF CALIFORNIA 225 



part of the Mississippi Valley region, and in the Cretaceous of the 

 same regions. 



The McCloud limestone of California represents nearly the whole 

 Coal Measures, with only a small part of the upper division absent, 

 and it is all rather pure limestone with a thickness of about two 

 thousand feet. The Coal Measures section of Arkansas embraces 

 all but the uppermost part of the formation, and the thickness is 

 approximately twenty thousand feet, if we leave out the Poteau 

 group, which is probably higher than the top of the McCloud lime- 

 stone of California. This would indicate that it takes approximately 

 ten times as long for a foot of limestone to form as it does for a foot 

 of sandstone. 



A similar -conclusion may be drawn from a comparison of the 

 Cretaceous sections of the two regions; and here the West Coast 

 has an arenaceous section, while the Cretaceous rocks of the South- 

 west are in places entirely calcareous, a reversal of conditions from 

 those of the Carboniferous. 



The Cretaceous section of northern California shows a thickness 

 of about thirty thousand feet, all sandy, and evidently deposited in 

 shallow water in a synclinal trough, just as was the Coal Measures 

 sandstone of Arkansas and Oklahoma. The Cretaceous beds of 

 the Southwest, ordinary marls and chalky limestones, have, where 

 not mixed with sandy deposits, a thickness of about three thousand 

 feet. This again indicates that it takes about ten times as long for 

 the accumulation of a foot of limestone to accumulate under ordinary 

 conditions as it does for a foot of sandstone. 



Of course the thicknesses vary in different parts of the same region, 

 and at best are only rough estimates; also we cannot be sure in widely 

 separated regions whether we have exactly the same geologic units 

 represented in the sections compared. Also it is not at all likely that 

 all limestones or all sandstones are laid down at even approximately 

 the same rates. Still the agreement of the figures is too great to be 

 accidental. 



If we accept this ratio of ten to one for rates of formation of sand- 

 stone and limestone, we have a means of estimating the relative 

 length of time consumed in laying down the rocks of the various 

 formations, even when their lithologic character is different. Thus 



