382 PEOCEEDIlN^GS OF THE GEOLOGICAL SOCIETY. [April 22, 



that the green slate contains local deposits of segregated iron as 

 crystallized pyrites. 



An example of green slate, converted from the purple in contact 

 with a dyke of diabase, Dinorwic Quarries, Llanberis (Analysis No. 

 52), contained 



Protoxide of iron 5-o68 



Sesquioxide 3"133 



Sulphur 0011 



Under the microscope the protoxide of iron appeared to occur as 

 chlorite. The production of great masses of green slate can there- 

 fore be explained by a simple chemical process, viz. the partial 

 reduction of the sesquioxide to a state of protoxide, and the separa- 

 tion of a small proportion as bisulphide rearranged as distinct crys- 

 tals. The green banding of the Cambrian slates, and the produc- 

 tion of the large uniform masses of green, were therefore not only 

 due to independent causes, but occurred at different times, — the 

 banding and blotching before the slate was cleaved ; and some of the 

 green slate was converted from the purple at the time of the intru- 

 sion of the greenstone dykes, which Professor Eamsay regards as of 

 Post- carboniferous age. 



10. On the Diseoloratioii of JRecl Beds hy Lime and Magnesia. — 

 The absence of red beds from strata that are interstratified with 

 calcareous bands, and the general absence of calcareous matter from 

 red beds, must be noticed in connexion with the following examples 

 of secondary variegation, which seem to be connected -with the pre- 

 sence of carbonate of lime. 



De la Beche, at page 53 of vol. i., Memoirs of the Geological Sui-vcy, 

 notices that many of the gTcen and blue bands in the red marls of 

 the Old Red Sandstone of Herefordshire were found to be calcareous, 

 or half- developed cornstones. "With this exception, the large extent 

 of non-calcareous r^cZ rock of the Old Eed Sandstone forms a striking 

 contrast with the great thickness of Upper Silurian dun-cohured 

 beds that underhe it, and in which tbe calcareous element pre- 

 vails. This is expressed in fig. 51, exhibiting an entire absence of 

 red beds, from the top of the Ludlow rock to near the base of the 

 Weulock shale, where a reddish-puiple bed appears. 



This fact is not accounted for by the absence of ferruginous mat- 

 ter, as the Wenlock and Ludlow dun-coloui'cd shales contain as much 

 iron as the Devonian red beds above, and the purple bed below. It 

 seems, however, to be directly correlative with the presence of car- 

 bonate of lime. In Shropshire the proportion of lime gradually in- 

 creases upwards, from the purple bed as a minimum, at the base, to 

 the massive limestone as a maximum, at the summit of the Wenlock 

 beds. 



xin analysis (N"o. 51) of this purple bed at Minton, near Little 

 Stretton, shows that it contained 



Protoxide of iron 0-72 '\ 



Sesquioxide 7'70 I Metallic iron 6-37, 



Insoluble oxides 0'7o J 



and about 7|- per cent, of carbonate of lime. 



