OLD RED SANDSTONE IS A MODE OF GREYWACKE. 87 



The old red sandstone, about which so much has been written and 

 SO little understood, is a greywacke, colored red by the accidental 

 admixture of oxide of iron. In Monmouthshire, the relations of 

 red sandstone with greywacke, and the passage of one rock into the 

 other, may be distinctly observed ; the connection also with the 

 lower gritstone, under the mountain limestone, may be plainly traced. 

 Here, then, we have the mountain limestone with its ahernating beds 

 of grit, the red sandstone and the greywacke, evidently members of 

 the same formation ; and to make the connection more complete, 

 the red sandstone contains beds of limestone, which form the link 

 between the lower transition and the upper transition limestones. 

 This limestone is imperfect, being intermixed with siliceous parti- 

 cles ; it is of a greenish color, and hence called Gooseberry lime- 

 stone. The red sandstone also passes into claystone, which is as 

 well characterised as that of the Pentland Hills.* 



The old red sandstone possesses all the mineral characters of 

 greywacke, except the color, which is a quality that can never be 

 considered of importance, being chiefly derived from local or acci- 

 dental causes. The old red sandstone also occupies the geological 

 position of greywacke, and greywacke-slate, into which it passes 

 merely by a change of color. The principal reason why it has not 

 been generally recognised as belonging to the greywacke formation is, 

 that it has been frequently confounded with the red sandstone above 

 the coal formation ; some of the beds greatly resemble each other ; 

 and it is not yet clearly ascertained, whether the red sandstone in 

 some parts of England and Scodand be the old red sandstone or the 

 new. Until English geologists shall renounce their prejudices, and 

 place the old red sandstone and mountain limestone in the Transi- 

 tion Class, as greywacke, and transition limestone, every attempt 

 will be vain to identify this part of the geology of England with that 

 of the Continent : particularly as the Alpine limestone of foreign 

 geologists, is a very different formation from the transition limestone, 

 comprising the several formations of limestone above the coal strata, 

 and new red sandstone, or what the French call Grcs bigarre. 



Transition Limestone. — This is one of the most important of the 

 transition rocks : its mineral characters vary considerably, accord- 

 ing to the nature of the rocks with which it is associated ; it has gen- 

 erally a subcrystalline texture, and is more or less translucent on the 

 edges. From the degree of hardness which it possesses, it will take 

 a good polish : most of the colored marbles are transition limestone. 

 The prevailing color is bluish grey, but it is sometimes red, brown, 

 or black : the lower beds of this limestone are often beautifully va- 



* From the quantity of oxide of iron and of red marie in some beds of the old 

 red sandstone, and from its passage into claystone, I am inclined to believe that 

 the red sandstone of Monmouthshire has been partly formed by the decomposition 

 of an ancient basaltic formation, which has become intermixed with greywacke. 



