SUBTERRANEAN WATERS. 667 



(2.) Mud-lumps, Mud-volcanoes. — The shallow waters within one 

 to three miles of the main channel or mouth of the Mississippi River 

 (see map, p. 659) are dotted with what are called mud-lumps, — con- 

 vex or low-conical elevations, sometimes 100 feet or more in diameter, 

 — showing their tops at the surface. They originate in upheavals 

 of the soft bottom. Once formed they discharge mud from the top, 

 which gives to the material of the low cone the structure of a volcanic 

 cone, the successive layers being, however, of mud, and but a fraction 

 of an inch thick. They finally collapse ; and then the cavity of the 

 cone sometimes becomes the site of a pool of salt-water, like the lake 

 in an extinct volcano. They are formed, according to Prof. E. \V. Hil- 

 gard (from whose description in the " American Journal of Science," 

 III., i., the facts here given are cited, and who adopts, in the main point, 

 the view of Lyell), through the pressure of the surface deposits on a 

 layer of mud which overlies the Port Hudson clay, or Champlain al- 

 luvium (p. 547). Some carbo-hydrogen gas is given out, arising from 

 the decomposition of animal or vegetable matters in the mud. 



3. Moisture confined ix Rocks. 



The amount of moisture in different rocks varies with their kinds 



and compactness of texture. 



In 1853, Durocher published some results of experiments with regard to the amount 

 of water contained in different crystallized minerals, giving, among them, -0028 to 

 •0269 per cent, for orthoclase, or common feldspar; -0127 for porphyry ; -0203 for euryte, 

 a feldspathic granite, etc. Delesse made further examination of rocks, in 1861, and 

 found the amount of moisture in coarse granite - 37 per cent., in euryte, 0-07; in 

 milky quartz, from a vein, 0*08; in flint, from the chalk at Meudon, 0*12 ; in a com- 

 pact Tertiary limestone (Calcaire grossiere), 3*11; in chalk, from Meudon, nearly 20 

 per cent.; in a quartzose sandstone (gres de Fontainebleau, near Meudon), 2 - 73. Hunt, 

 in some experiments, the results of which were published in 1865, obtained for the 

 amount of moisture absorbed, after drying at a temperature between 150° F. and 200° 

 F. : for Potsdam sandstone, three specimens, 2 # 26 to 2-71 percent. ; other three, 6-94-9-35 ; 

 for Trenton limestone, 0*32 to 1*70, the former for a black variety ; for the Chazy rock, 

 an argillaceous limestone, 6'45 to 13-55 ; a crystallized dolomite, of the Calciferous for- 

 mation, four specimens, 1*89 to 2*53; two other specimens, 5"90 to 7*22; for the Medina 

 argillaceous sandstone, two specimens, 8-37 to 10-06. J 



The facts, as first suggested by Sremann, early in 1861. and after- 

 ward at more length by Delesse, show that the thickening of the 

 supercrust, by the addition of sedimentary beds, has been attended by 

 the withdrawal of water from the oceanic and other superficial basins. 

 The metamorphism of strata has expelled this moisture, to a large 

 extent, from the beds thus altered, yet not wholly. The average 

 amount, in granite, syenyte, porphyry, and all Archaean rocks, is not 

 over 0*06 per cent. ; while in other rock formations it may be 2*5 



1 Durocher, Bull. Soc. Geol., x., 431, 1853; Delesse, ibid., xviii., 64, 1861; Hunt, 

 Amer. Jour. Sci., II., xxxix., 193. 



