302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1944 
actually entombed in the accumulation of sediments. It must not be 
destroyed by oxidation or devoured by the marine i peeveneens that 
normally feed upon such materials. 
There are two common environments pedis recurring in earth 
history in which organic matter, falling to the bottom of the sea, is 
effectively preserved for burial in the accumulating sediment: seas 
into which fine muds pour so rapidly that the stagnant bottom waters 
are too foul to permit the presence either of oxygen or of marine 
scavengers; and “desiccating” seas, those land-locked bodies of water 
all but cut off from the ocean proper, which are subjected to con- 
tinuous evaporation so intense that they become highly concentrated 
and the various salts nop ye dissolved in sea water are precipitated, 
settle out, and accumulate as “evaporites”—limestone, dolomite, salt, 
anhydrite, etc.—on the sea floor. ‘The waters of such seas become so 
salty that no life and very little oxygen are found in them, except 
in the surface layer which is diluted by rainfall and by constant or 
periodic inflow of fresh sea water from the adjacent ocean. 
When we survey the earth for evidence of conditions in the past 
which would best fulfill these specifications for rich and extensive 
petroleum resources, our attention is soon drawn to the unstable belts, 
covered much of the time by shallow seas, which lies around the mar- 
gins of the main continental platforms, between them and the great 
oceanic deeps. We note particularly the shallow depressions in the 
earth’s crust, which throughout much of the earth’s history have sep- 
arated the several continents at their points of closest approach. 
The best known of these troughs or depressed segments between the . 
continental masses is the region now occupied in part by the Persian 
Gulf, the Mediterranean, Red, Black, and Caspian Seas, lying between 
the continents of Africa, Europe, and Asia; another conspicuous basin 
occupied by land-locked seas is the site of the Gulf of Mexico and the 
Caribbean Sea between the continents of North and South America 
in the Western Hemisphere; a third is the shallow island-studded sea 
lying between the continents of Asia and Australia in the Far East. 
Through one geologic cycle after another these intercontinental de- 
pressions have been filled with shallow, land-locked seas, teeming with 
marine life, into which sediments poured rapidly from the land on all 
sides. Frequently, too, these depressions have been the sites of “des- 
iccating” seas. The earth’s crust beneath them is unstable or mobile 
and yields readily to stresses. Altogether these depressed zones 
between the continents seem admirably constituted to serve as natural 
reservoirs for the petroleum resources of the earth; and as soon as we 
look for petroleum in these regions we find abundant evidence of its 
presence. 
