CONDITIONS OF DEPOSITION 233 



not reach that stage, or there will Ije salt deposits above the gypsum. 

 'I'herefore the water will have to have a depth of more than 5,400 feet 

 when the last gypsum is deposited. 



Beds of gypsum 40 to 50 feet thick occur in many places, but seem to 

 be veiT limited in extent, and tliey often grade laterally into deposits of 

 wide extent, 10 feet or less in thickness. 'i4ie literature on gypsum 

 places little emphasis on the small extent of the thick deposits, but a 

 careful study of the sections given in various publications indicates the 

 patchiness. The thick Eed Bed gypsum deposits of Wyoming are rarely 

 more than 1 square mile in extent and occupy a small area compared to 

 the total. A rough estimate would be 100 square miles of 40 to 50 foot 

 gypsum and about 2,000 square miles of 10 -foot. If 11% feet were de- 

 ])osited over the entire area, the thicker beds could originate by currents 

 shifting gypsnm along the bottom and filling up the deeper depressions. 

 All of the thick deposits noted by the Avriter in the Red Beds were related 

 to the thinner deposits, as indicated in figures 1 and 2, and this is con- 

 clusive proof that the waters were not much deeper above the thick 

 deposits than above the thin. The extra depth of water required for 40- 

 foot l)eds over that for 10-foot beds is about 9,000 feet, if the precipita- 

 tion is from water five times as concentrated as normal sea-water, and it 

 follows that if the extra thickness was from direct precipitation the beds 

 should be depressed some 9,000 feet below the 10-foot beds. Deposits 

 10 feet thicker than the surrounding beds would require an extra depth 

 of water of about 3,000 feet. These relations are shown to scale in fig- 

 ures 1 to 4. 



If tlic 10-foot beds were deposited and the waters then gathered in 

 smaller pools, ihc gypsum might be redissolved and carried in by stvoaius. 

 Sli-('iiiiis llowiiiL:' over gypsniu bods c;irrv more calcium sulphate in soln- 

 lion than oi'dinary streams, Init do not carry enough to add rapidly to 

 the in(lose(l waters. Tn Clarke's "Data of Geochemistry" the highest 

 lalcinin snlpiiate content given for any stream water is about ! part to 

 1.000 by weight (using all of the Ca in the water in forming CaSO^) 

 for the Santa Maria River, 25 miles above Santa Maria, California.* If 

 such water came in fast enough to balance 5 feet of evaporation per year, 

 1 foot of gypsum would be supplied every 400 years; but calcium sulphate 

 makes up only about .4 of the material in solution in this water, and a 

 larger volume of other salts would probably be deposited than of gypsum. 

 The sediments carried in by the streams would probably amount to sev- 

 eral times as much as the materials carried in solution and the gypsum 

 would make up only a small part of the total deposit. In this connection 



" Data of peochomisti-v. Hull. 4;il. I'. S. CJpoI. Siirv.. p. 70. 

 Win -Bull. Okoi,. Soc. Am.. Vol. :^6, 1014 



