eh ae 
- 
Parr VIL Sect. ii. § 2.] LAVAS AND TUFFS. 565 
attains its greatest development among the volcanic eruptions of 
Tertiary time. Instead of mere local lenticular patches, these 
sheets lie piled over each other sometimes to a depth of several 
thousand feet, and frequently cover areas of many thousand square 
miles. Among the Paleozoic rocks of Scotland remnants of such 
ancient volcanic plateaux occur in the Old Red Sandstone (hills of 
Lorne) and Carboniferous systems (Campsie Fells and hills above 
Largs), where they consist chiefly of consecutive sheets of different 
porphyrites rising into long terraced tablelands. The regularity of 
thickness and parallelism of these sheets form conspicuous features in 
the scenery of the districts in which they occur. 
It is chiefly basaltic rocks, however, that in all parts of the 
world have escaped in fissure eruptions and now build up vast 
voleanic plateaux. The fragmentary Miocene plateaux of the 
British Islands, the Faroe Islands, and Iceland; those of the Indian 
Deccan and of Abyssinia, and the more recent basalt floods which 
have closed the eventful history of volcanic action in North America, 
are notable illustrations of this type of structure. Beds of tuff, 
conglomerate, gravel, clay, shale, or other ‘stratified intercalations 

ONE MILE 
Fic. 293.—SectTion oF INTERCALATED D1ABASE (TOADSTONE) IN CARBONIFEROUS 
LimEsToNE, DERBYSHIRE (B.). 
a a, Toadstone; b b, Limestones ; c, Millstone grit; f/f, Faults, 
occasionally separate the sheets of basalt. Layers of lacustrine clays, 
sometimes full of leaves, and even with sufficiently thick masses of 
vegetation to form bands.of lignite or coal, may also here and there 
be detected. But marine intercalations are rare or absent. There 
can be no doubt that these widely extended sheets of basalt were in 
the main subaérial outpourings, and that in the hollows of their 
hardened surfaces lay lakes and smaller pools of water in which the 
interstratified sedimentary materials were laid down. The singular 
persistence of the basalt-beds has often been noticed. The same 
sheet may be followed for several miles along the magnificent cliffs 
of Skye and Mull. Mr. Clarence King believes that single sheets of 
basalt in the Snake River lava-field of Idaho may have flowed for 50 
or 60 miles. The basalts, however, so exactly resemble each other 
that the eye may be deceived unless it can follow a band without 
any interruption of continuity. 
§ 2. Fragmental, or Tuffs. 
While the observer may be in doubt whether a particular bed of 
lava has been poured out at the surface as a true flow or has con- 
! “ Geological Exploration of 40th Parallel,” i. p. 593. 
