THE LIAS MARLSTONE OF LEICESTERSHIRE. 
127 
A point of some interest that arises out of the consideration 
of the mineral character of the Marlstone Rock is the question 
of its coloration. Originally grey or greenisli-blne, this rock 
becomes, when exposed to the prolonged action of the air (or of 
water containing oxygen), of a rusty brown colour; coincidently 
with tins change in colour, the carbonate of iron becomes 
converted into the hydrated ferric oxide. There appears to be 
good deal of doubt as to what gives the grey tint to the 
unweatliered stone. Dr. Sorby, with many others, supposes 
that the (bluish) green colour in this class of rocks is due to 
the presence of glauconite—an earthy mineral, having the 
composition of silicate of iron with some magnesia and water, 
and that the blue colour is given by the phosphate of iron. 
Prof. Judd, following Ebelman, holds that the blue colour in 
many oolitic limestones is due to a small quantity of sulphide 
of iron distributed through the rock mass, and ascribes the 
grey tint of the unweatliered Northamptonshire ironstone, and 
therefore, also of the Leicestershire marlstone, to this substance. 
The green colour he ascribes to either the silicate or the 
phosphate of iron. The bisulphide of iron is certainly 
present in small quantity in the Tilton stone, but it seems 
rather difficult to understand how this material could give a 
blue colour to the rock. A small amount of the silicate of 
iron is present in the rock, and perhaps also a little phosphate 
of iron. The simplest explanation would be to ascribe the 
original grey colour of the marlstone to the carbonate of iron, 
a material which, as a chemical salt, is white, but becomes 
grey on exposure, or possibly to this and the phosphate of 
iron which gives a more pronounced grey tint. The green 
colour of the partially weathered marlstone may, I believe, 
be looked upon rather as a transitional stage from the blue to 
the brown stone, than as an original tint. I consider that 
it may be explained by the formation, in gradually increasing 
quantities, during exposure, of particles of the yellow hydrated 
ferric oxide, disseminated through the unaltered grey particles 
of the rock, just as by mixing blue and yellow pigments 
together we get a green tint. 
(To be continued.) 
Rats. —A gentleman saw repeatedly the singular spectacle of three 
rats running abreast over his grounds. Noting their track, and their 
usual times, he shot them, and then found that the middle rat was 
blind, and that all three held a straw in their mouths. It is almost 
too good to be true.—“ Reminiscences,” by Rev. T. Mozley, M.A. 
Yol. I., p. 115. (London: Longmans, 1885.) 
