448 ORIGIN OF EOCK SALT. [Ch. XXil. 



sea, or into a lake, to form strata of red sandstone and red marl, pre- 

 cisely like the mass of the "Old Red" or "New Red" systems of 

 England, or those tertiary deposits of Auvergne (see p. 224), before 

 described, which are in lithological characters quite undistinguishable. 

 The pebbles of gneiss in the Eocene red sandstone of Auvergne point 

 clearly to the rocks from which it has been derived. The red coloring 

 matter may, as in the Grampians, have been furnished by the decom- 

 position of hornblende or mica, which contain oxide of iron in large 

 quantity. 



It is a general fact, and one not yet accounted for, that scarcely any 

 fossil remains are preserved in stratified rocks in which this oxide of 

 iron abounds ; and when we find fossils in the New or Old Red Sand- 

 stone in England, it is in the gray, and usually calcareous beds, that 

 they occur. 



The gypsum and saline matter, occasionally interstratified with such 

 red clays and sandstones of various ages, primary, secondary, and ter- 

 tiary, have been thought by some geologists to be of volcanic origin. 

 Submarine and subaerial exhalations often occur in regions of earth- 

 quakes and volcanoes far from points of actual eruption, and charged 

 with sulphur, sulphuric salts, and with common salt or muriate of soda, 

 In a word, £uch " solfataras " are vents by which all the products which 

 issue in a state of sublimation from the craters of active volcanoes ob- 

 tain a passage from the interior of the earth to the surface. That such 

 gaseous emanations and mineral springs, impregnated with the ingre- 

 dients before enumerated, and often intensely heated, continue to flow 

 out unaltered in composition and temperature for ages, is well known. 

 But before we can decide on their real instrumentality in producing in 

 the course of ages beds of gypsum, salt, and dolomite, we require to 

 know more respecting the chemical changes actually in progress in seas 

 where volcanic agency is at work. • 



The origin of rock salt, however, is a problem of so much interest 

 in theoretical geology as to demand the discussion of another hypoth- 

 esis advanced on the subject ; namely, that which attributes the pre- 

 cipitation of the salt to evaporation, whether of inland lakes or of 

 lagoons communicating with the ocean. 



At Northwich, in Cheshire, in the Upper Trias or Keuper, two beds 

 of salt,, in great part unmixed with earthy matter, attain the extra- 

 ordinary thickness of 90 and even 100 feet. The upper surface of the 

 highest bed is very uneven, forming cones and irregular figures. Be- 

 tween the two masses there intervenes a bed of indurated clay, trav- 

 ersed with veins of salt. The highest bed thins off towards the south- 

 west, losing 15 feet in thickness in the course of a mile.* The horizon- 

 tal extent of these particular masses in Cheshire and Lancashire is not 

 exactly known ; but the area, containing saliferous clays and sand- 

 stones, is supposed to exceed 150 miles in diameter, while the total 



* Ormerod, Quart. Geol. Journ., 1848, vol. iv. p. 211. 



