620 GEOLOGY OF BUTE. 



The Ascog coal measures may, however, be but a local deposit 

 laid over the older beds, and may not extend further along 

 the coast; and it is not improbable that the lower band only 

 of the sandstone next the slate may be of the Old Red forma- 

 tion, while the outer portions may correspond to the con- 

 glomerates of Brodick Bay. The strata are not exposed in 

 deep glens and high mountain sides, as in Arran ; and the 

 exploration of the beds is thus a matter of much greater 

 difficulty. 



Trappean Beds. 



134. The trap above referred to is a projection from the 

 principal mass above, which occupies a considerable area 

 inland towards Ascog Lake, and is upwards of 100 feet thick. 

 It is seen in the summit of the cliffs on either side, the line 

 of junction ascending rapidly as it retires from the shore. 

 These trap rocks at Ascog derive their chief interest from 

 being the repository of beds of lignite a substance so rare 

 in Scotland, that no well-marked beds occur on the main- 

 land, and but few in the other islands; and these in situa- 

 tions very difficult of access. 



The principal bed is situated in the face of the cliffs above 

 the road, a little to the south of Ascog mill, as shewn in the 

 annexed section (fig. 39), giving the various beds. 



The lowest bed resting on the sandstone is a small-grained, 

 rudely columnar greenstone; the junction is, however, con- 

 cealed. Over this is a trap-tuff with a base of greenstone, 

 and imbedded spherical lumps of the same substance. This 

 is followed by a bed of red ochre of coarse texture, traversed 

 by numerous black iron seams, which have doubtless been 

 produced from a change in the oxidation of the component 

 iron. Over this is the lignite bed : it is three feet thick, and 

 consists of hard stony coal, interstratified with a yellowish- 

 white shale, both being much intermixed with pyrites. The 

 coal has been so much altered throughout its whole thickness, 

 by the contact of the trap rock, that Mr. Rose of Edinburgh, 



