6 BULLETIN 102, PABT 3, UNITED STATES NATIONAL MUSEUM. 
portance of the Gulf sulphur deposits is the result of two factors, 
mutually important. These are the remarkable and unique manner 
in which the sulphur occurs, and the novel and highly effective 
process by which it is obtained without recourse to the costly methods 
of ordinary mining. 
THE GULF COAST DEPOSITS. 
The portion of Louisiana and Texas in which sulphur occurs is 
part of the Gulf Coastal Plain and is geologically a unit. The terri- 
tory is underlain by stratified rocks of great thickness, virtually hori- 
zontal. The even extent of these beds is interrupted here and there 
by a doming of the strata, producing in many places low surface 
knolls or mounds. These structural domes, as they are called, first at- 
tracted attention in the sixties of the last century in the course of 
prospecting for oil, when drill holes exposed their structure and con- 
tent. It was soon discovered that a comparatively large number of 
these crustal warpings are scattered through a coastal zone extending 
from Baton Rouge to Galveston and beyond, and that the upbowed 
beds form a cover for a remarkable assemblage of economic minerals, 
including petroleum, rock salt, and sulphur. 
These domes vary greatly in size, ranging in diameter from a few 
yards to 1| miles and extending to depths as great as several thou- 
sand feet. When explored by drilling, many of them have been 
found to consist of a cone-shaped core of solid rock salt, commonly 
associated with masses of cavernous limestone stained or saturated 
with petroleum; and almost invariably lenses of gypsum (calcium 
sulphate) and sulphur are present, the last usually impure or scat- 
tered. The upbowing of the strata around each of these occurrences 
suggests a common origin, namely, a crystallization of these immense 
bodies of minerals from supersaturated solutions rising from great 
depths and making room for deposition by the expanding force ex- 
erted by growing crystals. The source of these uprising solutions is 
thought to have been bodies of cooling igneous rocks lying miles below 
the surface; and the crystallization, once started, is deemed to have 
had sufficient power to lift bodily the mass of rock lying between the 
growing nucleus and the surface. Such, at least, is a reasonable 
explanation of these remarkable geological features ; and thus we see 
that, if these ideas are correct, both the salt and sulphur are ulti- 
mately of igneous origin, although the petroleum has probably gath- 
ered at a later date in the porous rock surrounding and surmounting 
the solid core. 
The proportions of petroleum, rock salt, and sulphur vary in the 
different domes, and hence the dominant constituent at any locality 
is the one for which mining operations are directed. The three, or 
even two, have in no place been worked in conjunction. The petro- 
