Ch. xxil] and rock salt. 343 



decomposition 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 j)reserv^ed in stratified rocks on 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 gTay, 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 and muriate of soda. 

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

 issue in a state of sublimation from the craters of active volcanoes, obtain 

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

 emanations and mineral springs, impregnated with the ingredients 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 iustrumentahty 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 hypothesis 

 advanced on the subject ; namely, that which attributes the precipitation 

 of the salt to evaporation, whether of inland lakes or of lagoons com- 

 municating with the ocean. 



At Northwich, in Cheshire, two beds of salt, in great part unmixed 

 with earthy matter, attain the extraordinary thickness of 90 and even 

 100 feet. The upper surface of the highest bed is very uneven, forming 

 cones and irregular figures. Between the two masses there intervenes a 

 bed of indurated clay, traversed with veins of salt. The highest bed 

 thins off towards the southwest, losing 15 feet in thickness in the course 

 of a mile."^ The horizontal extent of these particular masses in Cheshire 

 and Lancashire is not exactly known ; but the area, containing saliferous 

 clays and sandstones, is supposed to exceed 150 miles in diameter, while 

 the total thickness of the trias in the same region is estimated by Mr. 

 Ormerod at more tVan IVOO feet. Ripple-marked sandstones, and the 

 footprints of animais, before described, are observed at so many levels 

 that we may safely assume the whole area to have undergone a slow and 

 gradual depression during the formation of the Red Sandstone. The evi- 

 dence of such a movement, wholly independent of the presence of salt 

 itself, is very important in reference to the theory under consideration. 



In the "Principles of Geology" (chap. 27), I published a map, fur- 



* Ormerod, Quart, Geol. Journ. 1848, vol. iv. p. 277. 



