92 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



A^TsHre, between the villages of Dalmellington and Barr, there is 

 a copious development of similar sills, especially along one or more 

 horizons near the base of the Old Eed Sandstone. Garleffin Fell, 

 Glenalla Pell, Turgeny, and other heights are conspicuous promi- 

 nences formed of these rocks ; above the sills lie thick conglomerates 

 and sandstones on which the great porphyrite-sheets rest. In the 

 Dundee district, as already mentioned, the sills are of a more basic 

 character (andesites and diabases) ; they send veins into and bake 

 the sandstones among which they have been intruded, and are 

 sometimes full of fragments of such indurated sandstone, as may be 

 well seen on the northern shore of the Pirth of Tay, west of Dundee. 



A conspicuous characteristic of most of the volcanic tracts of the 

 Lower Old Red Sandstone in ' Lake Caledonia ' is the comparative 

 scarcity of contemporaneous dykes. In the band of acid sills 

 between Muirkirk and the Clyde a considerable number of dykes have 

 been mapped, which must be regarded as due to the same series of 

 movements and protrusions of the magma that produced the adjacent 

 sills. Throughout the length of the Southern Uplands dykes of 

 felsite, minette, lamprophyre, vogesite and other varieties, which 

 may also be connected with the volcanic phenomena of the Lower 

 Old Red Sandstone, not infrequently occur among the Silurian 

 rocks. On the Kincardineshire coast, south of Bervie, a number of 

 dykes of pink quartz-porphyry traverse the conglomerates and 

 sandstones. The coast south of Montrose displays some singularly 

 picturesque sections, where a porphyry dyke running through por- 

 phyrite lavas and agglomerates stands up in wall-like and tower-like 

 projections. On the shore at Gourdon, as well as at Cortachy and 

 Alyth, intrusive dykes of serpentine occur. One would expect to 

 meet with a network of dykes in and around the volcanic vents ; 

 but even there they are usually not conspicuous either for number 

 or size. Those in the great vent of the Braid Hills have been already 

 referred to, and allusion has been made to the possibility that, 

 besides the acid tuffs intercalated among the lavas of the Pentland 

 Hills, there may be sills and veins of felsite which it is difficult to 

 distinguish from parts of the tuffs. On North Black Hill, at the base 

 of the volcanic group of the Pentland chain, there is distinct 

 evidence of the intrusion of felsite. 



In the Ochil Hills also, groups of dykes of felsite and porphy- 

 rite may be observed, especially near the necks. They are fairly 

 numerous in the neighbourhood of Dollar. By far the most abun- 

 dant series yet observed, however, occurs beyond the limits of the 



