652 



SCIENCE. 



[N. S. Vol. III. No. 70. 



lar line. Small portions of Walden Ridge 

 and Sand Mountain appear in the extreme 

 southeastern corner of the district, these 

 being plateaus similar to the Cumberland 

 Plateau farther west. A small portion of 

 the Sequatchie Valley occupies the south- 

 eastern part of the district, with an altitude 

 of about 600 or 700 feet, while its north- 

 western portion is within the ' highland 

 rim,' a broad terrace surrounding the low- 

 lands of middle Tennessee and separating it 

 on the east from the Cumberland Plateau. 



Geology. — The rocks appearing at the 

 surface are of sedimentary origin, and in- 

 clude representatives of all the geologic 

 periods from Silurian to Carboniferous. 

 The Silurian formations, consisting of the 

 Knox dolomite, Chickamauga limestone 

 and Rockwood shale, occur only as narrow 

 belts in the Sequatchie Valley. The same 

 is true of the Devonian, which is repre- 

 sented by a single thin formation, the 

 Chattanooga black shale. The Carbon- 

 iferous formations occupy by far the larger 

 part of the district, the Fort Payne chert 

 and Bangor limestone forming the lower 

 portions of the plateau escarpments and 

 the highland rim, while the Lookout and 

 Walden sandstones, belonging to the Coal 

 Measures, form the summits of the plateaus. 



The geologic structure of the region is in 

 general extremely simple. The plateaus 

 and the highland rim to the westward are 

 underlain by nearly horizontal strata, while 

 Sequatchie Valley is upon a sharp, narrow 

 fold, the beds dipping downward on either 

 side beneath the adjoining plateaus. If the 

 rocks which have been eroded from the top 

 of this arch were restored, there would be 

 a ridge several thousand feet in height in 

 place of the present valley. In addition to 

 the folding which the strata have suflFered 

 along this line, they have been fractured, 

 and the beds on the east have been thrust 

 upward and across the edges of correspond- 

 ing beds on the west of the fracture, so that 



along the western side of the valley the 

 formations do not appear at the surface in 

 their normal sequence. 



Mineral resources. — These consist of coal, 

 iron ore, limestone, building and road stone 

 and clays. The Coal Measures occupy an 

 area within the district of about 500 square 

 miles. Not all of this area, however, con- 

 tains coal beds of workable thickness, while 

 some portions contain two or three work- 

 able beds. The lower beds, occurring in 

 the Lookout sandstone, are variable in 

 horizontal position, thickness and extent, 

 so that they can not profitably be worked 

 on a large scale ; but they have been 

 opened at many points, and supply an ex- 

 cellent fuel for local use. The Sewanee 

 seam, which is found in the Walden sand- 

 stone, from 50 to 70 feet above its base, is 

 the most important seam in the district. 

 It has an average thickness of 4 to 5 feet 

 over at least 80 square miles in the higher 

 portions of the plateau, and is extensively 

 mined for coking at Tracy and Whitwell. 

 The iron ore of chief importance is the red 

 fossil or ' Clinton ' ore, which occurs as a 

 regularly stratified bed in the Rockwood 

 shale. At Inman, in the Sequatchie Valley, 

 it attains a thickness of 5.5 feet and is ex- 

 tensively mined. 



FOLIO 18, SMAETSVILLE, CALIFORNIA, 1895. 



This folio consists of 4 pages of text, 

 signed by Waldemar Lindgren and H. W. 

 Turner, geologists, and Gr. F. Becker, 

 geologist in charge; a topographic sheet 

 (scale 1:125,000), a sheet of areal geology, 

 one of economic geology and one of struc- 

 ture sections. 



Topography. — The district of country rep- 

 resented lies between the meridians 121° 

 and 121° 30' and the parallels 39° and 

 39° 30', and embraces about 925 square 

 miles, comprising a part of the foothill re- 

 gion of the Sierra Nevada. The elevation 

 ranges from 50 feet above sea- level in the 



