384 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 22, 



This purple bed scarcely occupies a distinct stratigraphical hori- 

 zon, but passes upwards, with a blotchy interposition, into the great 

 mass of overlying dun-coloured beds. 



An analysis (No. 50) of these from between Harley and Much 

 Wenlock indicated the following composition : — 



Protoxide of iron 1 "80 ] 



Sesquioxide of iron 7*00 I Metallic iron 7*01, 



Insoluble oxides of iron . Oo5 J 



with about 9^ per cent, of carbonate of lime, besides calcareous 

 nodules of segregation. 



There is no material difference in the amount of the iron ; the 

 sesquioxide in the purple bed appears to exist as the anhydroiis 

 form, and that in the dun-coloured beds as the hydrous ; and the 

 proportion of protoxide to sesquioxide increases upwards with the 

 increase in the amount of calcareous matter, until in the blue beds of 

 the Wenlock limestone the iron exists principally as carbonate of 

 protoxide. The relation of this dun- colour to the presence of lime 

 is further supported by the fact, that where the calcareous Penta- 

 merus-zone appears between the purple bed and Llandovery conglo- 

 merate, it is accompanied by dun-coloured, and not purple shales, 

 and where the band of Woolhope limestone appears in Hereford- 

 shire on the horizon of the purple bed in Shropshire, the purple 

 colour is lost. The same principle ajDpears to hold good with respect 

 to the distribution of colour in the Trias of the east of France, a 

 sketch-section of which is given in fig. 52. 



In the neighbourhood of Luneville, near Nancy, the red colour of 

 the Marnes irisees disappears in descending to the Muschelkalk, the 

 calcareous bands being interstratified with grey marls ; and again, 

 when the Muschelkalk is passed, and the calcareous element goes 

 out, the red colour reappears in the Gres bigarre or Bunter. The 

 same relation of colour to calcareous matter is observable within the 

 Marnes irisees, and is illustrated in fig. 15 (PI. XII.), representing a 

 section at Enville near Luneville, in which the change of colour from 

 red to grey seems to have been induced by the presence of bands of 

 magnesian Hmestone. Immediately adjacent to these the marl is of a 

 uniform grey colour, and the underlying red colour begins to 

 come in as smaU isolated spots and lines ; these gradually increase 

 in proximity until they graduate, through a red ground freckled with 

 grey, into the mass of the uniformly red marl. 



The following analysis by Dr. Yoelcker gives the composition of 

 the interstratified calcareous layers, and of the red and grey parts of 

 the Marnes irisees at Enviile near Luneville. 



Analyses. No. 69. No. 70. No. 71. 



Eed marl. Grrey marl. Hard band. 



Water of combination 4-24 3-97 1*73 



Bisulpliide of iron 005 0-037 0-025 



Protoxide of iron 0-82 0-64 045 



Sesquioxide of iron 4-93 I'OO M3 



Alumina 1016 13-22 4-32 



*Lime 10-59 19-38 27'16 



No. 69. No. 70. No. 71. 



^ Equal to Carbonate of Lime 18-92 34-62 48-51 



