MINERALOGY OF THE REDHEAD. 355 



fine-grained sandstone *, and on the other, into a 

 mineral which may be termed compact clay. 



This mineral is of an ash-grey colour, some- 

 times also reddish-brown ; fracture compact, even, 

 passing into fine earthy ; fragments indetermi- 

 nately angular, rather blunt-edged ; translucent 

 on the edges ; dull ; streak light- coloured ; soft, 

 passing into semi-hard ; easily frangible ; adheres 

 a little to the tongue ; rather heavy. When 

 heated, and afterwards exposed to the air, it 

 crumbles down into a very coarse powder. This 

 substance, which is probably a combination of 

 silica and alumina, with a little iron, occurs in 

 globular- shaped masses, from half an inch, to 

 nearly a foot in diameter. These are compressed, 

 with an irregular margin, and in general an un- 

 even surface. They lie on their sides in the beds 

 of sandstone, especially where it passes into slate- 

 clay, and often also where it is soft and micaceous. 

 Although they occur insulated, yet they seem to 

 preserve a regular direction in the beds in which 

 they are included. The central parts of these 

 masses, are traversed by horizontal and vertical 

 rents, which do not make their appearance at the 

 surface. These rents are v/idest towards the 

 centre, and are in general filled with limespar ; 

 sometimes completely, at other times, only in 

 part. These balls appear to bear the same rela- 

 tion to sandstone, which the septarium is known to 

 bear to slate-clay in which it is usually imbedded. 



