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406 STRUCTURAL AND FIELD GEOLOGY 
plateau become progressively narrower and gradually reduced 
in height. At a later stage most of these mountainous 
segments may have disappeared, and only a few isolated cones 
and ridges or truncated pyramids may be left. Finally, every 
height may be levelled, and the old plateau be replaced by 
a plain of erosion. 
In the north-west of Scotland we have excellent examples 
of Relict mountains sculptured out of an ancient plateau of 
accumulation. In that region, certain old crystalline rocks 
(Archzan gneiss, etc.) had at a very early geological period 
been reduced to a base-level. The plain of erosion thus 
formed was then slowly submerged, and became in time 
covered with a great thickness of red sandstones. Long 
afterwards the whole region was re-elevated, thus forming a 
plateau of accumulation, the upper portion of which consisted 
of thick red sandstones resting on the surface of a plain of 
erosion composed of Archzan gneiss. So prolonged a period 
has elapsed since that epoch of elevation, that the red sand- 
stones have been largely removed, and much of the old plain 
of erosion has been re-exposed. Very considerable masses 
of the overlying red sandstones, however, still remain, forming 
isolated cone-like or pyramidal Relict mountains, such as 
Canisp, Soulvein, Stackpolly, and Coulmore, or more closely 
associated aggregates of similar shaped heights, such as the 
Torridon Mountains. 
Having glanced at the general character of Relict moun- 
tains which have been carved out of a plateau of accumulation 
—that is, out of an extensive elevated mass of horizontal or 
approximately horizontal strata—we may now shortly consider 
the character of the mountains which are chiselled out of a 
plateau of erosion. The relatively level surface of such a 
plateau is the result not of sedimentation but of denudation, 
A plateau of erosion may consist of many different kinds of 
rock, arranged in almost any way. In not a few cases, such 
plateaus represent the sites of vanished chains of Tectonic 
mountains. Externally they have a plain-like surface, inter- 
nally they frequently show all the confused and complicated 
structures which are characteristic of true mountains of 
upheaval. Plateaus of this kind are well represented in 
Europe. The Highlands and Southern Uplands of Scotland, 

