OLD RED SANDSTONE VOLCANOES. 299 



Silica . . . -63-0 

 Alumina . . .14*9 

 Iron Oxide . . 47 

 Lime .... 4*8 



Magnesia . . . 2'8 



Soda .... 4'O 



Potash . . . . l'9 



Loss .... 4 - o 



Thickness of Volcanic Rocks of the Lower Old Red Sandstone. 



In the Lower Old Red Sandstone country, between the base of the 

 highland mountains and the southern uplands, there were two lines 

 of contemporary volcanic vents from which vast lava streams and 

 accumulations of ashes were emitted. 



Notwithstanding denudation, more than 5000 feet of volcanic 

 rocks are measured at the northern end of the Pentland Hills, with- 

 out reaching the top ; and in the Ochill Hills more than 6000 feet 

 of similar rocks are seen without reaching the bottom. The Sidlaw 

 and Ochill Hills are composed of felstones and porphyrites, inter- 

 bedded tuffs and agglomerate, which extend for 60 or 70 miles, and 

 rise 2000 feet above the sea. 1 



Volcanoes of the Middle Old Red Sandstone. The Middle Old 

 Red Sandstone of Ayrshire abounds in interbedded rocks of volcanic 

 origin. They are often very slaggy and amygdaloidal, and are seen 

 in successive layers on the coast at Turnberry Point. The porphyrites 

 are generally separated from each other by thin beds of sandstone. 

 The Kirkoswald, Maybole, and Brown Carrick districts show the vol- 

 canic rocks of the Middle Old Red Sandstone, forming conspicuous 

 hills of pink porphyrites and dark compact dolerites ; and towards 

 the N.W. they sometimes pass into a kind of coarse sandy tuff. Vol- 

 canic rocks form the range of cliffs at Culzean, where the porphyrites 

 are dark green or purple. The porphyrite series, ends on the shore 

 near the heads of Ayr. In the Straiton and Dalmellington district 

 the Middle Old Red Sandstone reappears, with similar massive por- 

 phyrites to those of the Brown Carrick Hills. 



In this district the Lower Old Red Sandstone also contains dark- 

 purple, fine-grained, amygdaloidal porphyritic rocks. 



Further east, the middle of the Lower Old Red Sandstone is almost 

 entirely made up of purple and greenish slaggy and amygdaloidal 

 tuffs, with occasional bombs of porphyrite. 



The upper volcanic series is well seen in the district south of Irvine, 

 overlying sandstones with Ceplialaspis Lyelli ; and similar rocks occur 

 on the same horizon in Lanarkshire. 2 



Volcanoes of the Upper Old Red Sandstone. In the Upper Old 

 Red Sandstone of the Pentland Hills great consecutive sheets of fel- 

 stone, with occasional bombs and volcanic ash, are interbedded. The 

 larger beds, such as the felstone of Kips Hill, extend to the S.W. for 

 six or eight miles. The lowest bed, forming Warklaw Hill, is a com- 

 pact blue rock, which in its higher part becomes porphyritic and 

 amygdaloidal, and ultimately vesicular. It is succeeded by pale rose- 



1 " Carboniferous Volcanic Rocks of the Firth of Forth," Trans. Roy. Soc. 

 Edin., voL xxix. p. 441, and Brit. Assoc., Dundee, 1867, sec. p. 49. 



- Mem. Geol. Survey, Scotland ; Explanation of Sheets 13, 14, 15, 22, 23. 



