THE STKUCTURAL CLASSIFICATION 581 



sodium chloride, except at Belle Isle, Louisiana, where it is saturated 

 with oil. Galenite and sphalerite were also found at Belle Isle in a well 

 drilled on the center of the dome ; pyrites has been reported and gypsum 

 and sulphur are common accomj)animents. Borings made for oil and 

 sulphur at Belle Isle have discovered limestone, sulphur, and escaping 

 waters charged with hydrogen sulphide and sulphur dioxide, but the oil 

 drawn from these wells gives no indication of sulphur. 



In Spindletop dome certain wells have passed through oil rock and 

 gypsum and penetrated the salt core. The dolomitic oil-bearing rock is 

 estimated as 75 to 150 feet thick. Large cavities exist in the dolomite, 

 their size being estimated by large fragments shot from the wells. In 

 some cases drillers report that the tools have dropped several feet into 

 cavities which undoubtedly act as oil reservoirs. A test near the discovery 

 well at Spindletop entered gypsum at 1,200 feet, rock-salt at 1,650 feet, 

 and ended at 1,900 feet from the surface. While in some saline domes 

 oil has not been found in commercial quantities, it is known in great 

 quantity in many of them, and nowhere else in extreme southern Lou- 

 isiana and southern Texas. 



Apparent absence of salt in some domes. — In a few of the Gulf Coast 

 domes no rock-salt has yet been discovered. One of these instances is in 

 the Jennings field, at Prairie Mamou, Louisiana, where oil was found at 

 a depth of 1,800 feet. In the Welsh pool of Louisiana there is no topo- 

 graphic evidence of doming and no salt has yet been found; but, as at 

 Jennings, it is believed to exist. The best wells at Welsh are about 1,000 

 feet deep. At Sulphur, Louisiana, also known as Bayou Choupique, ooz- 

 ings of petroleum and gas led the Louisiana Oil Company to drill as long 

 ago as 1868. Clays, sands, and gravels were penetrated for 434 feet, and 

 then massive gray limestone 60 feet in thickness was encountered. Be- 

 neath this limestone alternate layers of pure sulphur and lim*estone were 

 found throughout a thickness of 260 feet, and still below were gypsum 

 beds with occasional layers of pure sulphur. Eock-salt is as yet unknown 

 at Sulphur, which is one of the great sulphur mines of the world. Since 

 the profitable working of the sulphur was undertaken, there has been no 

 serious attempt to exploit the oil, although heavy oil still flows from the 

 upper strata into certain old test wells. 



Distribution of saline domes. — Since the saline domes of the Gulf 

 coast may not be conspicuous in the surface topography, and since the 

 surrounding country is flat and without rock outcrops, the question is 

 frequently asked whether anything can be done in those fields toward 

 reliable predictions of the localities of occurrence of oil. While few at- 

 tempts to do this have been made in a systematic way, there is no doubt 



