168 



MAGNESIAN LIMESTONE. 



It may deserve notice, that the red sandstone generally occupies 

 the depressions in the more ancient strata, or what were once deep 

 valleys, and also fills up hollows on the surface of ancient rocks, as 

 represented in Plate III. fig 4. a a. Now, as these depressions and 

 hollows were, originally, filled up when the surface was under the 

 ocean, and are now raised some hundred feet above its present level, 

 without any apparent disturbance, this fact proves, that there were 

 two elevating causes acting at different epochs, — the first violent and 

 transitory, which tilted up the lower beds ; the second, more exten- 

 sive, but more gradual in its operation, which upheaved the whole 

 country above the ocean, and formed islands and continents. 



Magnesian Limestone. — The geological position of this rock is 

 over the lowest beds of new red sandstone; but where this is want- 

 ing, it lies unconformably over the regular coal formation : see Chap. 

 VIII. It is covered by the middle and upper series of new red 

 sandstone. 



The dolomite found in primitive and transition rocks has been be- 

 fore described ; it is commonly white, or light grey and granular. 

 That in the secondary strata has generally a dark brown or a yel- 

 lowish-brown colour : it contains a variable proportion of magnesia, 

 sometimes more than fifty per cent. 



The presence of magnesian earth, in the proportion of nearly one 

 half, in certain limestones, is a fact that strongly militates against the 

 theory, which ascribes the formation of all limestone rocks to animal 

 secretion ; unless it shall be found that magnesian earth is contained 

 in the shells and exuviae of marine animals. I believe no analyses 

 of shells or coral have yet been made, in order to ascertain the pres- 

 ence of magnesia as one of their constituent elements. Should mag- 

 nesia be found in the exuvise of certain orders of marine animals, 

 and not in others, it would not only favour the opinion that limestone 

 was of animal origin, but might also explain the cause of the alterna- 

 tion of beds of magnesian limestone with beds of common limestone, 

 in the same mountain. Or should some shells of one species con- 

 tain magnesia, and others none, it would prove that, under different 

 circumstances, the same animal might form its shell of different con- 

 stituent parts. 



Professor Sedgwick is inclined to derive the magnesian limestone 

 from the debris of beds of mountain or transition limestone which 

 contain magnesia ; but many beds of the magnesian limestone, above 

 the coal formation, have as much the character of original rocks as 

 the beds of transition limestone, and the difficulty is not removed by 

 this hypothesis ; for it still remains to enquire, from whence did the 

 mountain or transition limestones derive their magnesia ? Von Buch 

 ascribes the change of the common limestone into dolomite in the 

 Tyrol, to the action of volcanic rocks and volcanic vapours contain- 

 ing magnesia ; but this opinion is not likely to obtain many support- 

 ers. Can the magnesia found in some of the chalk rocks in England 



