Geological magazine . circles. At depth beneath the upraised strata is found a great plug ofcrj^stalline salts which in some cases has burst right through thestrata and reached the surface. The chief salts are sodium chloride,gypsum, and dolomite, which occur in ascending order. It is supposedthat these have been piecipitated from hot saturated solutionscij-culating below, and that the domes have been raised by the forceof crystallization. The domes apparently occur at places of weakness, such as theintersection of faults. Porous sandstones in such domed Tertiarystrata form ideal sites for the
Geological magazine . circles. At depth beneath the upraised strata is found a great plug ofcrj^stalline salts which in some cases has burst right through thestrata and reached the surface. The chief salts are sodium chloride,gypsum, and dolomite, which occur in ascending order. It is supposedthat these have been piecipitated from hot saturated solutionscij-culating below, and that the domes have been raised by the forceof crystallization. The domes apparently occur at places of weakness, such as theintersection of faults. Porous sandstones in such domed Tertiarystrata form ideal sites for the accumulation of oil, and have yieldedgreat quantities. 4. Strata upraised hy Igneous Intrusions.—Probably this case ismore common than is yet proved, for many domes and anticlinals mayat depth be underlain by laccolitic or other intrusions (though if this 22 T. 0. Bo^icoyth — OatUiies of Oilfield Geology. be so it would still be difficult to discriminate between the fold andthe intrusion as to cause and effect).. Cretaceous. Volcanic 4. Oil accumulation due to igneous doming. In Mexico there are numerous volcanic necks which have burstthrough Tertiary strata, raising them and producing a dome-likestructure around each core of igneous rock, and thus forming veryfavourable conditions for the concentration of oil in the sands (Fig. 4). 5. Inclined Strata intersected hy Dykes.—In the same area dykesintersect the strata and sometimes act as dams to the oil ascendingan inclined sandstone bed (Fig. 5).
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1864
