548 
ELEMENTS OF GEOLOGY. 
(see section, p. 74), the beds of massive trap, and the tuffs 
composed of volcanic sand and ashes, dip regularly to the 
south-east or north-west, conformably with the shales and 
sandstones. 
But the geological structure of the Pentland Hills, near 
Edinburgh, shows that igneous rocks were there formed dur¬ 
ing the newer part of the Devonian or Old Red’’ period. 
These hills are 1900 feet high above the sea, and consist of 
conglomerates and sandstones of Upper Devonian age, rest¬ 
ing on the inclined edges of grits and slates of Lower De¬ 
vonian and Upper Silurian date. The contemporaneous vol¬ 
canic rocks intercalated in this Upper Old Red consist of 
feldspathic lavas, or feldstones, with associated tuffs or ashy 
beds. The lavas were some of them originally compact, oth¬ 
ers vesicular, and these last have been converted into amyg- 
daloids. They consist chiefly of feldstone or compact feld¬ 
spar. The Pentland Hills, say Messrs. Maclaren and Geikie, 
afford evidence that at the time of the Upper Old Red Sand¬ 
stone, the district to the south-west of Edinburgh was for a 
long while the seat of a powerful volcano, which sent out 
massive streams of lava and showers of ash, and continued 
active until well-nigh the dawn of the Carboniferous period."^ 
Silurian Volcanic Rocks.- —It appears from the investiga¬ 
tions of Sir R. Murchison in Shropshire, that when the Lower 
Silurian strata of that country were accumulating, there were 
frequent volcanic eruptions beneath the sea; and the ashes 
and scoriae then ejected gave rise to a peculiar kind of tufa- 
ceous sandstone or grit, dissimilar to the other rocks of the 
Silurian series, and only observable in places where syenitic 
and other trap-rocks protrude. These tuffs occur on the 
flanks of the Wrekin and Caer Caradoc, and contain Silurian 
fossils, such as casts of encrinites, trilobites, and mollusca. 
Although fossiliferous, the stone resembles a sandy claystone 
of the trap family.f 
Thin layers of trap, only a few inches thick, alternate in 
some parts of Shropshire and Montgomeryshire with sedi¬ 
mentary strata of the Lower Silurian system. This trap 
consists of slaty porphyry and granular feldspar rock, the 
beds being traversed by joints like those in the associated 
sandstone, limestone, and shale, and having the same strike 
and dip.J 
In Radnorshire there is an example of twelve bands of 
stratified trap, alternating with Silurian schists and flag- 
* Maclaren, Geology of Fife and Lothians. Geikie, Trans. Royal Soc. 
Edinburgh, 1860-1861. 
t Murchison, Silurian System, etc., p. 230. t Ibid., p. 212. 
