STRUCTURAL TERMINOLOGY 



the Cincinnati and Nashville structures are the earliest of the broad, 

 gentle uplifts studied in the United States, they probably should be 

 taken as types, and definitions should be fashioned after their character- 

 istics. At the completion of the present study of the uplifts and depres- 

 sions of the central stable region of the United States, nothing undesirable 

 is recognized in taking the Cincinnati and Nashville structures as types 

 for the United States, if a little latitude in characteristics is tolerated. The 

 terms in this report will be used as follows: 



An arch is a gentle, broad uplift with an evident width of 25 to 200 

 miles and a length conspicuously greater than the width. The structural 

 relief may amount to 10,000 feet or more between a bed at the top of the 

 arch and one of similar age at the bottom of the adjacent basin, but the 

 dip of the beds will generally not exceed 100 feet per mile. The struc- 

 tural relief may have been acquired in part by subsidence of the adjacent 

 basins at a greater rate than the arch area, so that the arch may actually 

 only at times have been an emergent landmass. 



A dome is a gentle, round or elliptical uplift of arch proportions. It 

 usually occurs along an arch and expands the arch locally. This regional 

 structural meaning of dome must be distinguished from the usage in con- 

 nection with igneous rock masses (Rice, 1940) and from the much 

 smaller oil- and gas-producing structures such as salt domes or plugs. 



Swell 



Schuchert ( 1923 ) used the term swell to mean all large, domed areas 

 within the nuclear part of the continent. Rucher ( 1933 ) defined a swell as 

 "an essentially equidimensional uplift without connotation of size or 

 origin." In discussing the structures of the United States the terms arch 

 and dome are sufficient for all broad gentle uplifts, to which the term 

 swell would generally apply, and therefore it has not been necessary to 

 use swell in the following pages, and no attempt to define it further will 

 be made here. 



Uplift and Upwarp 



Uplift and upwarp are used for a wide variety of structural elevations, 

 and, therefore, should be reserved as noncommittal terms in regard to 



size, shape, internal structure, and origin. If it is desired to distinguish 

 the two, uplift might be conceived as implying both small and lai 

 round and elongate elevations, with sharp and gentle variations; whan 

 upwarp would imply simply broad and gentle archings. Nfo pre c edent 



can be cited for this distinction, but a perusal of the literature leaves me 

 with the impression that this is the most general usage, l'rovmc iallv, how- 

 ever, uplift may mean a rather definite type of structure. I will use the 

 terms only in case I am in doubt about the nature of a structural el 

 tion, or desire to use them as synonyms of structures being discussed in 

 order to eliminate repetition. 



Basin 



Rucher (1933) uses the term basin in a structural sense to mean any 

 essentially equidimensional depression without connoting size or origin, 

 and then gives the Michigan basin as an example. Swell is his antithetical 

 structure of basin. Since the drill in several places has extensively ex- 

 plored the subsurface distribution of the stratified rocks of the continent. 

 a number of downwarps have become firmly entrenched as basins in the 

 literature. Some embrace more than a large state, and some are of county 

 size. Some are fairly elongate, and most all have axial directions. Some 

 are troughlike or furrowlike. It has not proved disturbing in compiling 

 the present review to have basin used in this loose sense, and I believe the 

 variations in meaning will be evident to the student, so there is little urge 

 to attach limitations to the term. The word basin is applied a thousand 

 times each day by petroleum geologists in many variations of meaning, 

 and it would appear unwise to attempt standardization. 



Coal basins have not proved to be the same as oil basins or water 

 basins in several places, and also the extent of the commercial materials 

 has not coincided with the greatest thickness of the strata and. therefore, 

 the greatest depression. It seems to me that the major geological features 

 should govern the choice of a geographic name, rather than the distribu- 

 tion of an economic deposit of little relative volume. 



The site of maximum subsidence during an epoch, period, or era may 

 not coincide with that of a later one, and some confusion has resulted in 

 the meaning of the term basin in certain areas. This is particularly true 



