A474 CO. 8S. Garnett—Alteration of Shales in Derbyshire. 
ferruginous substances of the shale—although almost unnoticeable 
in the normal regions—have been completely changed to the red 
oxide which remains in a very finely divided state. In some of the 
material the redness is very vivid, but more than 4% of ferric oxide 
is seldom present. 
As is well known, the Limestone Shales frequently contain, 
especially towards their base, thin beds of black, earthy limestone ; 
the blackness being, of course, due to carbonaceous (bituminous) 
matter as in the case of the shales, which, normally, are black at 
this horizon. A thin slab of this limestone which occurred about 
the margin of the region of alteration, was itself also bleached in 
a similar manner. It was grey in colour, almost free from car- 
bonaceous matter, and contained faint streaks and spots of red 
oxide of iron—evidently resulting from the oxidation of ferruginous 
matter which previously existed in the rock. 
Small phosphatic nodules also occur in some abundance in the 
Limestone Shales of this district ; normally, they are somewhat hard, 
and in colour uniformly almost black throughout. Some are 
calcareous and contain about 63°%, tricalcic phosphate, about 20°% 
calcium carbonate, 3°% silica, and some carbonaceous matter. To 
the unaided vision they show no definite structure, although having 
a somewhat crystalline appearance. Others are almost free from 
carbonates; they are harder and consist of compact amorphous 
calcium phosphate (tricalcic phosphate about 63°/%) with 20° of 
silica, and are rather more black in colour. It might be here 
mentioned that much of the calcium phosphate in the calcareous 
nodules is seen by microscopical examination not to consist of the 
amorphous substance, as is the case in the non-calcareous ones and 
phosphatic nodules generally, but it exists as minute crystalline 
spherulitic masses of definite material—presumably a variety of 
apatite. 
The nodules which occur in the altered region are themselves 
also altered in a manner exactly similar to that of the shale. Those 
which have been completely altered are white throughout, and 
are soft and friable. In these the carbonaceous matter has been 
completely removed, and also the carbonates and some phosphate. 
There remains a white mass containing the siliceous matter, 
varying amounts of phosphate and also a little sulphate. Although 
completely altered nodules are not plentiful, two types of incomplete 
alteration occur abundantly. In one type—that of the originally 
calcareous nodules—they are soft and friable; the carbonates 
have been removed, also most of the carbonaceous matter, but 
not yet much phosphate. On being broken they present a speckled 
appearance. In the other type—that of the non-calcareous nodules, 
which in the normal condition are very hard and dense—these 
nodules occurring in the region of alteration, show an unaltered 
interior ; the bleaching action has been very complete so far as it 
has proceeded, but this is often only to a depth of about $ or jin. 
and the zone of alteration is comparatively sharply defined. 
