70 PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



been the most vigorous in the whole region. Their chief vents lay 

 towards the south-west in the neighbourhood of Stirling, where the 

 lavas, agglomerates, and tuffs discharged from them reach a thick- 

 ness of not less than 6500 feet, without revealing their bottom. 

 From that centre the lavas range continuously for sixty miles to the 

 north-east until they overlap those of the Porfarshire group. How 

 far they stretched south-west cannot now be ascertained, for they 

 have been dislocated and buried in that direction under the Carbo- 

 niferous formations of the Midland Valley. While the most active 

 vents were situated in the Stirling district, others rose at irregular 

 intervals along the length of the lake towards the north-east. The 

 numerous bosses around Dundee, of which Dundee Law may be 

 taken as a type, possibly mark a cluster of vents in that neighbour- 

 hood. 



3rd. A third group of active orifices lay nearer the southern 

 margin of the lake, and had its centre on the southern outskirts of 

 the city of Edinburgh. There was a large vent on the site of the 

 Braid Hills, from which lavas and tuffs were thrown out to a total 

 depth of at least 7000 feet. These materials form now the chain of 

 the Pentland Hills. They can be followed south-westwards for ten 

 miles, until they dwindle down and die out among conglomerates 

 and sandstones. Their north-easterly prolongation is concealed 

 under the Carboniferous strata which overlap them unconformably. 



4th. Another distinct group of small volcanoes had its centre 

 about 25 miles to the south-westward on the same line as those of 

 Edinburgh. In no part of the basin can the isolation of the 

 diflferent volcanic clusters be so impressively observed as in the area 

 to the south-west of the Pentland Hills. On the one hand, the 

 lavas and tuffs from the Edinburgh vents die out, and, on the other, 

 as we follow the conglomerates south-westwards a new volcanic series 

 makes its appearance, rapidly increasing in the number and thick- 

 ness of its members until, where traversed by the Eiver Clyde, it 

 occupies the whole breadth of the tract of Old Eed Sandstone, and 

 must be several thousand feet in thickness. These lavas and tuffs 

 may have come mainly from one or more vents near to the site 

 of the well-known eminence of Tin to. Erom that centre they 

 extend north-eastwards for about sixteen miles before they finally 

 die out, while in a south-westerly direction they can be traced only 

 for about ten miles, when they are lost under the Carboniferous 

 formations which are faulted down against them. 



5th. Among the high bleak muirlands on the confines of the three 



