O. 8. Garnett—Alteration of Shales in Derbyshire. 475 
In the case of the actual shale also there are two exactly similar 
types of incomplete alteration. Very occasionally only hard, 
compact shale lamine are found which contain a small unaltered 
"portion at their centres ; the altering agency not having penetrated 
so far. The other, entire but incompletely changed, condition 
is common at the margins. As has been mentioned, at the greatest 
exposure the beds lie in a confused manner ; portions of unaltered 
black beds have become involved in the altered region, which 
seems to have consisted of surface beds to a depth of several feet. 
Alteration continued to take place after the portions of unchanged 
beds came in contact with the changed ones, but only to a small 
extent, for at the margins of the black portions the decarbonization 
can be readily seen to have gradually proceeded a short distance 
inwards. In this case the shale was soft and readily permeable, 
and shows a gradual transition in greyness from the edge of the 
mass to several inches inwards, where visible action is lost. A 
similar transition may be traced in the actually altered beds, though 
very much more gradually, as they pass from the region of alteration. 
The phenomena of the occurrence very closely correspond with 
those of burning; i.e. combustion with the complete removal of 
all carbonaceous matter and oxidation of ferruginous substances. 
The nearest known occurrence of igneous rock may be expected 
at about 150 feet below the top of the limestone, that is, about 
180 feet below the surface at this place; it was cut through in the 
old Mill Close shaft, about 4 mile approximately north-east of the 
occurrence under consideration, but it has been found to be a 
contemporaneous flow, and could not be the cause of the burning. 
Spontaneous combustion arising from the oxidation of pyrites 
would seem to be the explanation of the occurrence. However, one 
or two somewhat peculiar features were noticed, and some work was 
conducted under the supervision of the author in an endeavour to 
obtain as much elucidation as possible of available evidence. 
In the case of the mass of normal (black) shale in the altered 
zone, the change is seen to have proceeded distinctly further along 
the planes of bedding than against them. The mass in question 
happens to be pushed well into the altered region, but it almost 
retains the normal horizontal position of the bedding ; a small crack 
had formed almost vertically near its centre, and in this, a com- 
mencement of apparently the same change in the shale had taken 
place, diminishing in extent as it passed downwards exactly as 
would be expected if the change had been brought about through 
the action of percolating water. 
The question arises, what weathering agent could produce the 
vast chemical changes involved? From the changes which have 
taken place in the calcareous phosphatic nodules there is a suggestion 
of slight acidity—by sulphuric acid, but on consideration the more 
conspicuous changes would seem to be that of very powerful oxidizing 
action; indeed, a ‘“‘ wet combustion”. Hydrogen peroxide or 
