280 FLORIDA GEOLOGICAL SURVEY-SEVENTH ANNUAL REPORT. 
PORTSMOUTH FINE SAND. 
The Portsmouth fine sand, in its typical development, consists of 
a dark-gray to black fine sand, which has a decided loamy feel, 
owing to its high organic-matter content. The black surface ma¬ 
terial passes abruptly into a light-gray, incoherent fine sand at 
depths varying from 5 to 15 inches and averaging about 8 inches. 
This is underlain by a dense, compact hardpan layer, usually at a 
depth of 18 to 24 inches. The hardpan stratum is characteristically 
dark brown or black in the upper part, but it quickly grades into 
a copper-brown, rusty-brown, or “coffee-grounds” color, and be¬ 
comes less compact with depth. This layer is usually from 4 to 8 
inches in thickness, and the dark, dense portion comprising the upper 
part averages 3 inches. Below the hardpan stratum the material 
to a depth of 36 inches or more varies from a light-brown to a 
light-gray, incoherent fine sand, usually saturated. 
The depth to the hardpan layer is extremely variable. It may 
occur immediately below the black surface soil or at any depth 
within the 3-foot section, and in places it is entirely absent. In 
the latter case the soil grades from the black surface material into 
a gray fine sand, which continues with uniformity of color and tex¬ 
ture to a depth of 3 feet or more. Though all these variations may 
occur within a comparatively small area, it is quite exceptional to 
find the hardpan absent over a tract of any considerable extent. The > 
areas having no hardpan in the 3-foot section comprise a phase in¬ 
termediate between the typical Portsmouth fine sand and the Ports¬ 
mouth fine sandy loam. 
The origin of the hardpan layer in the subsoil is not definitely 
known, though it is believed to be caused either by filtration or pre¬ 
cipitation of organic matter, carried downward as fine particles or 
in solution by the percolating water, at or near the mean level of the 
water table. The hardpan stratum has frequently been ascribed to 
the precipitation of iron salts leached from the surface, and it is a 
popular belief that it is rich in iron, but laboratory tests show that 
the content of iron is not very much greater than in other parts of 
the soil section, while the content of organic matter is high. Water 
solutions show a brownis'h color similar to that of the streams of the 
region, which indicates that a part of the cementing material is 
soluble. 
The Portsmouth fine sand is of greater extent and more widely 
distributed than any other type, areas of this soil occurring in all 
