6 THE LICHENS OF PERTHSHIRE 
t the 
of dolerite—a coarse-grained variety of basalt. The Sidlaw and 
Ochil Hills are almost entirely composed of volcanic rocks of Old 
Red Sandstone age. They consist of beds of lava and volcanic 
eR ser aa ee dolerite, tuff-aglomerate, &c.—most of which 
land area of Perthshire we have a large series of very ancient 
rocks, many of them metamorphic, and 
quartz-schist, slate, limestone, &c., with intrusions of basalt, 
porphyrite, felsite, diorite, and granite. i 
great boundary fault and proceeding northwards, we have first a 
narrow band of shales, grits, and cherts, supposed to belong to 
e Upper Cambrian system. They extend from the west of 
Aberfoyle to the east of Callander. Then we have the Leny 
schistose grit and the Birnam and Aberfoyle slates. Northwards 
again there is a broad band of quartz-schist, which extends from 
Loch Chon to the Forest of Alyth, and forms the rugged moun- 
tain masses of Ben Venue, Ben Ledi, Ben 
Next comes the mica-schist rock of Ben More, which, 
laid, however, with moraine gravel and beds of peat. Granite 
is also found on the mountains to the south-east of Look Ericht, — 
