136 FLORIDA GEOLOGICAL SURVEY-THIRD ANNUAL REPORT.
where the "Trail Ridge" forms part of the boundary. A narrow strip along this part of the county exceeds ioo feet in elevation. With this exception practically all parts of this county lie below the IOO foot contour line, while much of the area lies below the 25 foot contour line.
The elevations in Nassau and Duval Counties have been obtained from various sources. An important line of levels extending from Trout Creek across Nassau and Duval 'Counties in a southwesterly direction, made during the summer of 19o9 in connection with a preliminary survey for a ship canal across Florida, were kindly made available for this purpose in the office of the United States Engineers at Jacksonville. Similar surveys made by the same office in 1879 supplied elevations from Fernandina to Maxville and at various points along the St. Marys River.' .n addition much information as to elevations has been obtained f.om the profiles of the several railroads crossing this section, particularly the Seaboard Air Line from Jacksonville to Maxville, the Florida East Coast from Jacksonville to Mayport and the Atlantic Coast Line from Jacksonville to the St. Marys River.
From Jacksonville westward the rise in elevation as shown by the profile of the Seaboard Air. Line Railway is very gradual to a point three miles west of Jacksonville where an elevation of 27 feet is reached. From this summit the elevation drops off slightly, the elevation of Cedar Creek being 17 feet. Beyond Cedar Creek the elevation rises more rapidly. Marietta station is approximately 6o feet above sea. The summit of this rise is reached two miles west of Marietta where the elevation is 94 feet. White House station is 82 feet above sea. Beyond McGirts Creek one and one-half miles an elevation of 91 feet is reached. From this point there is a very gradualslope to Baldwin, this latter place being 86 feet above sea. iSouth from Baldwin the contour rises in general reaching an elevation of 93 feet at Maxville and ioo feet one-half mile beyond the county line.
The line of levels run by United States Engineers extends from Trout Creek passing just to the south of Brandy Branch station, Or Bryceville postoffice. The summit elevation in Nassau and Duval Counties along this line occurs about four miles northeast of Brandy Branch, where an elevation of 90 feet is recorded.
WATER-BEARING FORMATIONS.
The deeper wells in Duval County reach and terminate in the Vicksburg Limestone. This is known to be the case at Jackson*Annual Report for the Chief of Engineers for 188o, pp. 973-I01O.