chap, xxxvi STRUCTURE OF THE VOLCANIC PLATEAUX 
183 
know, these outbursts were subserial. At least no trace of any marine 
deposit has yet been found even at the base of the pile of volcanic rocks. 
Sheet after sheet of lava was poured out, until several thousand feet had 
accumulated, so as perhaps to fill up the whole depression, and once more to 
change entirely the aspect of the region. But the volcanic period, long and 
important as it was in the geological history of the country, came to an end. 
Tt, too, was merely an episode during which denudation still continued active, 
and since which subterranean disturbance and superficial erosion have again 
transformed the topography. In wandering over these ancient lava-fields, 
we see on every hand the most stupendous evidence of change. They have 
been dislocated by faults, sometimes with a displacement of hundreds of feet, 
and have been hollowed out into deep and wide valleys and arms of the sea. 
Their piles of solid rock, hundreds of feet thick, have been totally stripped 
off from wide tracts of ground which were once undoubtedly buried under 
them. Hence, late though the volcanic events are in the long history of the 
land, they are already separated from us by so vast an interval that there 
has been time for cutting down the wide plateaux of basalt into a series of 
mere scattered fragments. But the process of land-sculpture has been of 
the utmost service to geology, for, by laying bare the inner structure of 
these plateaux, it has provided materials of almost unequalled value and 
extent for the study of one type of volcanic action. 
I. NATURE AND ARRANGEMENT OF THE ROCKS OF THE PLATEAUX 
The superficial outbursts of volcanic action during Tertiary time in Britain 
are represented by a comparatively small variety of rocks. These consist 
almost wholly of basalts, but include a number of less basic rocks which may 
be classed as andesites. Many andesitic sheets, like the andesitic dykes, 
have been intruded into the basalts, and are really sills. 
Besides the lavas of the basaltic-plateaux there are intercalated deposits 
ot tuffs and breccias and large masses of agglomerate. A brief notice of the 
general petrography of the various constituents of the plateaux and their 
niode of occurrence will here be given. The intrusive bosses which have 
disrupted the superficial lavas will be discussed in subsequent chapters.. 
i. LAVAS 
1. Petrographical Characters 
(«) Basalts and Doleritcs . — In external characters these rocks range from 
coarsely crystalline varieties, in which the constituent minerals may be 
more or less readily detected with the naked eye or a field-lens, to dense 
black compounds in which only a few porphyritic crystals may be rnega- 
scopically visible. One of their characteristic features is the presence of the 
ophitic structure, sometimes only feebly developed, sometimes showing itself 
