1821.] Mr. Weaver on Floe tz Formations. 353 



the lower limestone formation, its thickness varies from 120 to 

 420 feet ; while at a greater distance, it has been found 800 feet 



thick. . . w 



The formation appears under different circumstances in ditter- 

 ent districts. In a great part of the forest of Thuringia, it 

 consists of uniform sandstone, beneath which is found a bed of 

 clay with gypsum and rocksalt ; and above the sandstone occurs 

 another bed of clay with interposed beds of marl, gypsum, rock- 

 salt, and bituminous layers. M. Heim states each of these beds 

 of clay to be 200 feet thick. In Mansfeld, on the other hand, 

 the formation consists of four principal members, variegated day 

 or clay marl, sandstone, slaty sandstone, and roestone, beside 

 other beds incidental to it, which will be noticed hereafter. But 

 the principal members may be said to be inseparable, for where 

 .me occurs, the others also are generally to be found ; and they 

 pass by insensible gradations into each other, being commonly 

 disposed in moderately thick beds, in indeterminate order. \et 

 when one or the other member (particularly the sandstone or 

 .lav) acquires an unusual thickness and predominates, the 

 others are still not wholly wanting. 



Principal Members. 

 Clan.— This, whether pure, or in the state of clay marl, is 

 generally brownish-red, thin and straight slaty, glimmering, and 

 tough, disposed in thin strata ; or crumbly and unstratified, m 

 great thickness. Commonly mixed with minute scales of mica, 

 or with fine sand, passing into slaty sandstone ; and sometimes- 

 it appears as indurated clay or claystone, with a flat conchoidal 

 fracture, of a brownish-red, greenish-grey, or mountain-green 

 colour. With the thick beds of red clay, as also with those of 

 the roestone and sandstone, there frequently alternate thin 

 layers of grey, yellow, green, and bluish clay. These also pass- 

 into slaty°sandstone and argillaceous sandstone, by an admixture 

 of mica and sand. The clay is commonly more or less calca- 

 reous, whatever its colour may be. No petrifactions have been 



found in it. . . 



Sandstone— This, which forms the most distinguished and 

 predominant member of the whole formation, consists of several 

 varieties, possessing an argillaceous, calcareous, or siliceous 

 cement ; but principally of those whose cement is of the nature 

 of the first two ; all of them, however, alternate with each other, 

 raid also in endless diversity with clay, slaty sandstone, roestone, 

 and calcareous beds. 



The argillaceous sandstone is white, yellow, or grey, brownish- 

 red, or variegated. The first three kinds are fit for the purposes 

 nf the architect and statuary, consisting, for the greater part, of 

 tine or minute grains of quartz, of an equal consistence, with an 

 argillaceous cement', and some mica. Hence it has beea called 



New Series, vol. ii. 



2 A 



