MIDDLE DISTRICT OF SCOTLAND. 



629 



sometimes a greyish-black. The red-coloured 

 variety is oftentimes variegated with stripes, 

 layers, and circular portions of a green colour. 

 Some varieties of this rock are so highly impreg- 

 nated with carbonate of lime, that they may be 

 considered as impure kinds of slaty marl. It 

 passes into claystone, and sometimes also into clay- 

 ironstone; 



3. Claystone, This mineral occurs in beds that 

 tary in thickness from an inch to several yairds. 

 It alternates with slate-clay, and also with rW 

 sandstone, and with some other rocks which are 

 subordinate to that formation. 



4. Clay -ironstone. It occurs in layers, or in ir- 

 regular or globular-shaped masses, generally in- 

 eluded in slate-clay. A beautiful transition is to 

 be observed from the slate- clay into the ironstone : 

 at one extremity of the series is the pure slate-clay, 

 at the other the petfeet clay-ironstone ; and the 

 middle part of the series, is a substance interme- 

 tliate between the slate- clay and the ironstone. It 

 follows from this fact, that all the phenomena ex- 

 hibited by the clay ironstone, such as globular 

 forms^ cotemporaneous veins, &c. are of the same 

 general nature with those of the slate-clay, and that 

 if slate-clay owes its characters to deposition from 

 a state of aqueous solution, the same must have 



VOL. 11. s s 



