140 PROCEEDINGS OF THE GEOLOGICAL SOCIEir. 



seams of stoue can be traced through it, such as the Hurlet Limestone, 

 already referred to, and the Index Limestone. Only at the north end, 

 where the volcanic mass is thickest and the surf ace-exposures of rock 

 are not continuous, has it been impossible to subdivide the mass by 

 mapping intercalations of sedimentary strata across it. It would 

 thus seem that, even where the thickest and widest accumulations 

 gathered round the puys, they formed low flat domes, rather than 

 hills, which, as subsidence went on and the tuff-cones were washed 

 down, gradually sank under water, and were buried under the 

 accumulating silt of the sea-floor. I shall offer some further details 

 regarding the structure of the puys and their erupted lavas and 

 tuffs when I give a rapid resume of the characters of some 

 illustrative districts wherein these phenomena can best be studied. 



3. Sills and Di/Jces. — One of the characteristic features of Central 

 Scotland is the great number and often the large size and extra- 

 ordinary persistence of the sills which have been injected among 

 the Carboniferous strata. The precise geological age of these intru- 

 sive sheets cannot, of course, be more exactly defined than by 

 stating that they are younger than the rocks which they traverse. 

 I have, however, been led, for the following reasons, to connect the 

 great majority of them with the puys, though some of them are 

 certainly of far later date. 



In the first place, they certainly do not form part of the pheno- 

 mena of the plateaux, where the sills are in great measure acid 

 rocks, while those now under consideration are much more basic. 

 But not only so, they appear for the most part on platforms later 

 than the plateau-eruptions. A most remarkable illustration of this 

 statement is furnished by the chain of the Campsie Eells, where, on 

 the north side, among the Calciferous Sandstones which emerge from 

 under the lavas of this plateau, many intrusive sheets and bosses of 

 felsite may be seen, while on its southern side come the great 

 basic sills which, from Milngavie by Kilsyth to Stirling, run in 

 the Carboniferous Limestone series. A similar contrast may be 

 observed in Kenfrewshire between the felsitic sills below the 

 plateau-lavas south of Greenock and the basic sills above these 

 lavas in the Carboniferous Limestone series around Johnstone and 

 Pa,isley. 



In the second place, the basic sills often occur in obvious con- 

 nexion with the vents or bedded lavas and tuffs of the puy series. A 

 conspicuous and well-known example of this dependence is supplied 

 b}^ the intrusive sheets of Salisbury Crags, Heriot Mount, and the 



