452 



NATURE 



[July 27, 19 16 



SOUTHERN GEORGIA AND ITS HYDRO- 

 GRAPHY.'^ 

 ALONG the eastern coast of North America, com- 

 mencing at Long Island and passing southward 

 through Virginia, North and South Carolina, Georgia, 

 and Florida, there lies a broad tract of country known 

 as the Atlantic Coastal Plain. This plain, which 

 also extends round the northern part of the Gulf of 

 Mexico, where it is distinguished as the Gulf Coastal 

 Plain, is a region of low elevation, with a relatively 

 gentle seaward slope. Part of it passes through and 

 embraces 35,000 square miles of the southern half of 

 the State of Georgia, and this constitutes the purview 

 of an extremely interesting and informative report 

 issued by the United States Geological Survey, from 

 which the following particulars are gleaned. 



Although characterised as a plain in comparison 

 with the mountainous country behind, the expanse 

 under consideration is not entirely without topograph- 

 ical features and contrasts. There are hilly and 

 broken areas, especially towards the north, but these 

 do not rise above the general level, and their summits 

 present an even skyline. The plain lends itself to 

 subdivision into six physiographical districts, the 

 nature of which may be to a large extent gauged from 

 their designations, viz., the Fall-Line Hills, the 

 Dougherty Plain, the Altamaha Upland, the Southern 

 Lime-Sink Region, the Okefenokee Plain, and the 

 Satilla Coastal Lowland. The lithological com- 

 ponents of these belts are principally sands, clays, and 

 marls, with, subordinately, limestones and sandstones. 

 The former are largely unconsolidated, and have 

 undergone little alteration since their original deposi- 

 tion. The sediments are representative of the Lower 

 Cretaceous and subsequent systems, and include the 

 Ripley Formation, about 950 ft. thick, of grey, cal- 

 careous, and micaceous sand, and sandy clay, and the 

 Midway Formation, about 400 ft. thick, of ferru- 

 ginous sand, with local beds of white clay, and 

 fossiliferous limestone and calcareous quartzite. The 

 Cretaceous deposits immediately and unconformably 

 overlie a basement of crystalline rocks believed to be 

 pre-Cambrian. 



The mean annual rainfall of the plain is about 

 49 in., and the quantity absorbed by the soil and 

 rocks is roughlv estimated at 90 to 95 per cent, of 

 the total. If nearly 60 per cent, of the rainfall be 

 assumed to be lost by evaporation and 4 or 5 per cent, 

 escape as run-off or flood-flow, there remains about 

 35 per cent, to form the underground water supply; 

 but much of this is not actually utilisable, on account 

 of the depth to which it descends. 



Although several of the cities in central Georgia, 

 such as Augusta and Macon, obtain their water sup- 

 plies from adjacent rivers, the majority of the in- 

 habitants have to depend upon supplies drawn from 

 artesian wells, of which there are probably some 700 

 or 800 in active operation. These wells range in 

 depth from 100 to 1000 ft. All the Cretaceous forma- 

 tions contain water-bearing strata, as also the Eocene 

 and Oligocene series of the Tertiary system. The 

 Quaternary svstem furnishes non-artesian water, 

 which is tapped by shallow borings. Such water, on 

 account of its high content of organic matter in many 

 cases, is not generally suitable for domestic use. 



A large number of analyses of the ground waters 

 have been made, and from^ them it is computed that 

 relatively few contain normal carbonate (CO,), while 

 the presence of hvdrogen-sulphide gas and of excessive 

 amounts of iron' is reported in waters from all the 

 formations. The gas imparts an objectionable odour 



1 "Underground Waters of t^e Coastal Plain of Geo-g^a." Bv L. W 

 Stephenson. T. O. Veatch, and R. B. Polo, (Water Sunnly Paper No 3.41 ) 

 Pp 530. with photographs, maps, and diagrams. Washington: United 

 States Geological Survey, icjis-) 



Tjn OAlQ VOL. Q71 



in certain instances and gives rise to corrosion in 

 boilers and mains. The iron, which in a number of 

 cases exceeds three parts per million, is then per- 

 ceptible to the taste, and tends to produce stains in 

 fabrics which are washed in it. B. C. 



HARDNESS AND CRITICAL COOLING 

 VELOCITIES OF STEELS. 

 T'HE maximum cutting hardness of pure carbon tool 

 •'• steel is achieved by water-quenching. With the 

 introduction of Mushet's special steel, engineers ob- 

 tained a material which was called '"self-hardening,' 

 because it did not require to be water-quenched in 

 order to bring out its maximum cutting hardness. It 

 was sufficient for the tool to be cooled from above a 

 certain critical temperature in air. The modern high- 

 speed tool steel falls into the same class of materials, 

 the chief difference from Mushet's special steel being 

 that the "lip" or "nose" of the tool requires to be 

 actually melted and then cooled in an air blast if the 

 maximum cutting hardness is to be obtained. Stated 

 in general terms, therefore, the rapid-cutting tool of 

 to-day is gas-quenched as contrasted with the carbon 

 tool, which is water-quenched. 



Various theories of the mechanism of the above 

 changes are held, and therefore the research by Prof. 

 C. A. Edwards, of the University of Manchester, 

 assisted by J. N. Greenwood and H. Kikkawa, re- 

 cently presented to the Iron and Steel Institute, on 

 some very remarkable properties of a chromium steel, 

 is to be welcomed in that it throws valuable light on 

 what are to some extent matters of dispute. This 

 steel contained 6-15 per cent, of chromium and 063 

 per cent, of carbon, the balance being iron, except 

 for impurities unavoidably present in small amounts. 

 By suitably varying the initial temperature and the 

 cooling velocity of this steel by air-quenching, Brinell 

 hardness numbers varying from 194 to 700 could be 

 obtained. Such a material therefore falls within the 

 category of self-hardening steels in the sense that 

 water-quenching is not required to harden it. On the 

 other hand, it was found that unless a certain critical 

 velocity of cooling was exceeded depending on the 

 initial temperature this steel did not harden. In this 

 sense, therefore, the steel does not appear to be self- 

 hardening. On this point the authors say : — " Whilst 

 with the chromium steel the cooling rates which pro- 

 duce hardening are extremely slow as compared with 

 those which are obtained in the hardening of steels 

 by quenching, the two operations are fundamentally 

 the same. In other words, a given rate of cooling, 

 which might be regarded as slow for carbon steels, 

 really constitutes quenching in the case of some special 

 alloy steels." The authors have further found that 

 the hardening of the steel coincides with the presence 

 of large quantities of martensite, and a diminution 

 in the magnitude of the carbide thermal change. The 

 maximum hardness was obtained when the thermal 

 transformation had been entirely prevented, and when 

 this was accomplished the steel was purely martensitic 

 in structure. The following table gives the connec- 

 tion between the initial temperature and the cooling 

 velocities between 836° C. and 546° C. which suppress 

 the carbide change : — 



Initial temperature 



°C 



. 860 



90S 



960 



1029 



1 147 



1200 



1267 



Cooling velocities 



1 36 



2 24 



3 o 



4 o 



6 o 



7 o 



8 56 



H. C. H. Carpenter. 



