472 Geihie — Volcanic Bocks of Great Britain. 



sistence of volcanic phenomena in a limited area. This remark 

 applies either to our country viewed as a whole, or to many of its 

 minor districts. These islands are but a small fragment of the 

 surface of the globe, and yet we see that volcanic action has been 

 rife here from Lower Silurian up into middle Tertiary times. But 

 the fact comes before us still more impressively when we discover 

 it in the geology of a single county, or even of a parish. Take, as 

 an illustration, the neighbourhood of Edinburgh within a radius of 

 ten miles from the town. First and oldest comes the long range of 

 the Pentland and Braid Hills, consisting of a mass of bedded 

 igneous rocks in a middle series of the Old Eed Sandstone. These 

 old lavas reach a thickness of 4,000 or 5,000 feet. Next in chrono- 

 logical order are the Calton HiU and lower portion of Arthur's Seat, 

 which mark the continuance of volcanic action (though in a lessened 

 degree) into the Lower Carboniferous period. The Carboniferous 

 rocks for miles around these hills are full of the traces of contem- 

 poraneous volcanos, sometimes in the form of sheets of tufi" marking 

 the occuiTence of little detached tuff-cones, sometimes in wider areas 

 of tuff, basalt, and dolerite, where a group of minor volcanic vents 

 threw out showers of ash and streams of lava. To the east rise the 

 isolated Garlton Hills, which date from before the Carboniferous Lime- 

 stone ; westwards, scores of little basaltic crags and rounded tuff- 

 hills mark out the lower Carboniferous volcanos of Linlithgowshire. 

 To the north the endless crags, hills, and hillocks of the Fife coast 

 contain the record of many eruptions from the middle of the 

 Calciferous Sandstones high up into the Carboniferous Limestone 

 group. Even the Coal-measures of that county are pierced with 

 intrusive bosses of trappean agglomerate which indicate the position 

 of volcanic vents, possibly of Permian age. The same or a more 

 recent date must be assigned to the later unconformable agglomerate 

 and basalt of Arthur's Seat. Nor is this the whole. Latest of all, 

 come innumerable trap-dykes, running with a prevalent east and 

 west trend, and cutting through all the other rocks. These, for the 

 reasons already stated, may, with probability, be assigned to a 

 Tertiary age. Here, then, in this little tract, about the size of a 

 small English county, there are the chronicles of a long series of 

 volcanic eruptions, beginning in the middle of the Old Eed Sand- 

 stone, and coming down to a time relatively so near our own as that 

 of the Miocene rocks. Nor is this by any means an exceptional 

 district. Illustrations of a similar persistence of volcanic action 

 may be gathered in many other tracts of equally limited extent. 



Another fact, which a general survey of the character of our 

 volcanic rocks soon brings before us, is that, as a whole, those of 

 earlier date differ distinctively in composition from those of more 

 recent origin. From the first traces of volcanic activity in this 

 country up to about the close of the Old Eed Sandstone or beginning 

 of the Carboniferous series, the interbedded (that is, contemporane- 

 ous) igneous rocks consist for the most part of highly feldspathic 

 masses, to which the name of clinkstone, claystone, compact feld- 

 spar, porphyry, hornstone, felstone, etc., have been given. In most 



