1858.] MOORE SILUEIAN ROCKS, PORTENCROSS. 3 



to the south of the disturbance, are pitched up to a right angle, 

 while to the north the}^ are elevated to about an angle of 50°. 



The breadth of this intruded mass is about 50 yards ; and I think 

 it impossible for any geologist acquainted with the aspect of the 

 Silurian rocks in the south of Ayrshire to doubt that this is of 

 Silurian age. In mineral constitution, in cleavage, in variety of 

 colour, in the violence they have been subjected to, they exactly 

 resemble Silurian rocks, and in all these respects they present the 

 strongest contrast to the Devonian rock with which they are in 

 contact. It also seemed to me on the spot equally impossible 

 to doubt that the Devonian beds to the north are the very bottom of 

 that series, as here developed. They can be observed for several 

 yards overlying the Silurians, — of the squeezes and contortions of 

 which they do not partake, having merely been tilted without 

 further derangement. The lowest bed is a dark-red sandstone and 

 conglomerate of small dark pebbles, apparently Silurian, but con- 

 taining no white quartz-pebbles. This continues along the shore, 

 traversed by a great dyke of greenstone, till we arrive at the ruins 

 of Portencross Castle, where the dark-red sandstone and conglo- 

 merate is covered by a very coarse conglomerate of pink and white 

 quartz -pebbles, of the size of a child's head. This is succeeded 

 by sandstone, with occasional layers of small white quartz -pebbles, 

 — the sandstone near the top being marly, and thus agreeing in 

 appearance with the sandstone first described, near Ardrossan. 

 It is cut through by a dyke of red porphyry, about 12 feet thick, 

 which has forced itself up, and spread itself over the sandstone, 

 forming the Hill of Goldenbeiry. This capping of porphyry, 

 judging by the eye, cannot be less than 300 feet thick. The 

 junction of the sandstone and porphyry is well seen in a per- 

 pendicular inland cliff; and the whole scene will well repay the 

 pains of a visit, from its picturesque beauty, independent of its 

 geological interest. The sandstones continue to dip north as far as 

 they are seen, until they are cut off by an arm of the sea. 



The Devonian beds immediately in contact with the two sides of 

 the Silurians are not identical : the dark-red conglomerate and the 

 coarse quartzose conglomerate, which are seen to repose on the 

 Silurians to the north, do not occur on the south ; and in their place 

 we find, as above stated, vertical beds of sandstone similar to that 

 which forms the base of Goldenberry Hill, and to that which forms 

 the whole coast to Ardrossan. It would appear that the elevation 

 of the Silurian rock has been attended by a fault ; and, while the 

 lowest beds of the Old Red Sandstone are brought up on the north, a 

 portion of the upper beds on the south side of the fault have been 

 thrown back to a perpendicular position. 



The nearest Silurian rocks to this spot as yet known are those in 

 the parish of Lesmahago, on the opposite or south side of the 

 Ayrshire and Lanarkshire coal-field, distant upwards of 30 miles 

 across the strike. These were described by Sir Roderick Murchison 

 in the 12th vol. of the Quart. Joum. Geol. Soc. In comparing the 

 two sections, it wiU be observed that in both cases the Devonian 



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