Ch. XXXIL] devonian VOLCANIC ROCKS. ^99 



tion explanatory of the structure of Forfarshire, already given (p. 48), 

 the reader will perceive that beds of conglomerate, No. 3, occur in 

 the middle of the Old Red Sandstone system, 1, 2, 3, 4. The peb- 

 bles in these conglomerates are sometimes composed of granitic and 

 quartzose rocks, sometimes exclusively of different varieties of trap, 

 which last, although purposely omitted in the section referred to, is 

 often found either intruding itself in amorphous masses and dikes 

 into the old fossiliferous tilestones, -No. 4, or alternating with them in 

 conformable beds. All the different divisions of the red sandstone, 

 1, 2, 3, 4, are occasionally intersected by dikes, but they are very 

 rare in JNTos. 1 and 2, the upper members of the group consisting of 

 red shale and red sandstone. These phenomena, which occur at the 

 foot of the Grampians, are repeated in the. Sidlaw Hills ; and it 

 appears that in this part of Scotland volcanic eruptions were most 

 frequent in the earlier part of the Old Red Sandstone period. 



The trap rocks alluded to consist chiefly of felspathic porphyry 

 and amygdaloid, the kernels of the latter being sometimes calcareous, 

 often chalcedonic, and forming beautiful agates. "We meet also with 

 claystone, clinkstone, greenstone, compact felspar, and tuff. Some of 

 these rocks flowed as lavas over the bottom of the sea, and enveloped 

 quartz pebbles which were lying there, so as to form conglomerates 

 with a base of greenstone, as is seen in Lumley Den, in the Sidlaw 

 Hills. On either side of the axis of this chain of hills (see section, 

 p. 48), the beds of massive trap, and the tuffs composed of volcanic 

 sand and ashes, dip regularly to the southeast or northwest, conform- 

 ably with the shales and sandstones. But the geological structure of 

 the Pentland Hills, near Edinburgh, shows that igneous rocks were 

 there formed during 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, resting on the 

 inclined edges of grits and slates of Lower Devonian and Upper Silu- 

 rian date. The contemporaneous volcanic rocks intercalated in this 

 Upper Old Red consist of felspathic lavas, or felstones, with asso- 

 ciated tuffs or ashy beds. The lavas were some of them originally 

 compact, others vesicular, and these last have been converted into 

 amygdaloids. They consist chiefly of felstone or compact felspar. 

 The Pentland Hills, say Messrs. Maclaren and Geikie, afford evidence 

 that at the time of the Upper Old Red Sandstone, the district to the 

 southwest 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 Period. — It appears, from the investigations of Sir R. 

 Murchison in Shropshire, that when the lower Silurian strata of 



* Maclaren, Geology of Fife and Lothians. Geikie, Trans. Royal Soc. Edin- 

 burgh, 1860-1861. 



