424 HISTORICAL GEOLOGY 



deformation comparable to that in North America folded, crumpled, 

 and metamorphosed the Ordovician strata at the close of the period. 

 In Scotland the folding was exceptionally severe, producing over- 

 turned folds and faults, in one locality thrusting strata ten miles 

 along a fault plane. 



In Europe, although the Ordovician often underlies the Silurian 

 unconformably, the disturbances which ushered in the latter appear 

 to have been slight. On both continents the important disturbances 

 took place where thick beds of sediment had accumulated. 



Petroleum and Natural Gas 



Conditions Favoring the Accumulation of Oil and Gas. — The inn 

 portance of the oil and gas industry is such that the essential features 

 of their geological occurrence demand attention. 



Petroleum and natural gas occur in varying quantities in all of the 

 fossiliferous rocks, from the Ordovician through the Tertiary, but oil 

 never occurs in paying quantities unless there is a porous stratum 

 overlain by an impervious one, in this respect resembling artesian 

 wells. In an artesian well, however, it is essential that the porous 

 stratum be open to the surface in order that the supply of water 

 may be replenished. In an oil well, on the contrary, if the porous 

 stratum reaches the surface the oil is lost by evaporation, since the 

 supply of oil comes from below. 



Oil and gas usually occur at or near the crest of broad anticlines 

 (p. 254) or other " reservoirs," where their further movement upward 

 is prevented, the oil moving up the porous stratum through the 

 water which permeates the bed, since oil and gas are lighter than the 

 former. If, however, water is absent from the porous stratum, the 

 oil will be at the bottom of the syncline and the gas in the anticline. 



One of the modes of occurrence common in the eastern United 

 States and Canada is shown in Fig. 395 A, in which the oil and gas 

 gradually move up the bed until (1) the anticline is reached. If the 

 stratum is saturated with water, the oil and gas will accumulate under 

 the anticline, but (2) if water is absent, the oil will accumulate in the 

 syncline (Fig. 395 B), while the gas passes on to the highest attain- 

 able point. 



(3) Oil is accumulated also when the strata are domed up, as in 

 Texas. The principle of the accumulation of the oil is the same as 

 in the anticline. (4) In Mexico there are numerous volcanic necks 



