1821.] Mr. Weaver on Floetz Formations. 341 



of the Quantocks in Somerset,* where they repose chiefly, if not 

 entirely, on transition tracts ; in the calcareous and calcareo- 

 magnesian conglomerates of the Mendips, being there frequently 

 metalliferous, producing galena, calamine, and iron ore ; in those 

 of the vicinage of Bristol, t of the environs of Tortworth, and of 

 the southern quarters of Monmouthshire and Glamorganshire ; 

 in all of which tracts they repose on the first floetz series ; and 

 such is also the general position of this formation in the north of 

 Endand.t 



D 



2. Loicer Limestone Formation of M. Freiesleben. 



Magnesian Limestone Formation.^ 



In Mansfeld, the lower portion of this formation is constant in 

 its order of position, wherever the continuity is preserved ; the 

 marl shale reposing on the weissliegende, and being covered by 

 the zechstein. But the members of the upper portion, lying 

 above the zechstein, observe no regular order of succession, 

 being generally disposed in a different order in different districts ; 

 sometimes also comprising one or more beds in one quarter, 

 which are wanting in another. 



The whole formation is distinguished by the presence of 

 gypsum, disseminated in the lower portion, but in mass in the 

 upper. A second characteristic of this formation is its general 

 tendency to a porous or cavernous structure. And a third is to 

 be found in its variable composition, as resulting from different 

 proportions of lime, alumine, silex, oxide of iron and bitumen, 

 differently intermixed and combined. It may also be remarked 

 of this formation in general, that its thickness is variable, not 

 only as a whole, but as respecting its different members. 



Lower Portion. 



Marl Shale, comprehending Copper Shale and Roof Shale. — 

 The bituminous, black or brown, marl shale, which, from its 

 frequent metalliferous, and in particular its cupriferous character, 

 has obtained the name of copper shale, varies much in different 

 quarters, and even in the same district, according to the relative 



■ See Mr. Horner's paper in Geol. Trans, vol. iii. p. 355, et seq. 



f See the papers of Dr. Bright, Mr. Warburton, and Dr. Gilby, in Geol. Trans. 

 vol. iv. p. 202—214. 



| 1 am not aware of beds or masses of trap, porphyry, or amygdaloid, being found 

 as a formation subordinate to this conglomerate and sandstone. The masses, adduced 

 as such in Devonshire, seem, on the contrary, to protrude from, and to belong to, the 

 transition tracts of that country, e. g. near Thorverton. (See the Rev. J. J. Conybeare 

 on the Red Rock Marl, in the Annuls of Philosophy, April 1821, and Mr. Greenough's 

 Geological Map). 



Is there any well authenticated instance of trap, porphyry, or amygdaloid, occurring 

 as a subordinate member of the second or third floetz series of formations ? 



§ We are indebted to Prof. Buckland for the first indication of the identity of the 

 English magnpsian limestone formation, and that which was denominated in Germany 

 the (irst floetz limestone by Werner. (See " Order of Superposition of Strata" 

 appended to the " Geology of England and Wales, by W. Phillips. MGS." in IS 18.) 



