242 Mr. H. Seeley on the Stratigraphical Position 



washed down from above ; and moreover, the darkest part of the 

 bed is the lowest. Nor is it likely that the decomposition of 

 nodules of pyrites diffused throughout its mass could have ob- 

 tained without leaving some marks both of longitudinal banding 

 and of centres of colour. Iron pyrites may have occurred 

 sparingly, as in many other beds ; but we must look for a more 

 diffused and finely-divided source of colour than the experience 

 of its occurrence in other cretaceous deposits would induce one 

 to believe could have here prevailed. I know of no such source 

 but the colouring matter of the Upper Greensand. This, as is 

 well known, consists of one-fourth of iron, which on decomposi- 

 tion passes into a different state of oxidation ; and that in this 

 process it does become red is very evident from the occurrence 

 of red patches at Blackdovvn. Thus we have a cause capable 

 of producing the effect, which is exactly such as all the other 

 evidence would lead us to look for. But there is another argu- 

 ment in the composition of the rock, which goes far to support 

 this idea of the origin of the colour. In Mr. Wiltshire's paper 

 it is stated to be — 



Carbonate of lime with a little alumina 82'3 



Peroxide of iron 6*4 



Silica 11-3 



Now, according to one analysis given by Dr. Fitton, the colour- 

 ing matter of the Greensand consists chiefly* of 



Alumina 1*75 



Protoxide of iron 6*25 



Silica 12-50 



This can scarcely be an accidental coincidence ; and when it is 

 remembered that the proportions of the components vary slightly 

 with the locality, it is impossible, on taking into consideration 

 all the previously considered circumstances, not to give great 

 weight to the fact of the identity between certain constituents 

 of the Hunst^on Limestone and those of the colouring matter of 

 the Upper Greensand. 



* 



Since the above was written, I have received from near Louth a 

 pale-coloured specimen of the red limestone, with the green 

 particles in an undecomposed condition. I had previously ob- 

 served at Hunst'on what I believed to be chloritic particles ; but 

 from their small size, and from decomposition having in most 

 cases set in, the identification was not sufficiently satisfactory. 



* Besides these ingredients, there is mentioned some potash ; but in 

 another analysis it was scarcely perceptible ; it is thus shown not to be 

 essential, and, as the comparison does not require it, has been omitted. 



