CHAP. V 
HISTORY OF OLD VOLCANIC VENTS 
75 
might seriously affect this regularity. The sedimentary formations, piled 
above each otlier to a great depth, and acquiring solidity by compression, 
might be thrown into folds, dislocated, upheaved or depressed. The buried 
volcanic funnels would, of course, share in the effects of these disturbances, 
and eventually might be so squeezed and broken as to be with difficulty recog- 
nizable. It is possible that some of the extreme stages of such subterranean 
commotions are revealed among the “ Dah'adian ” rocks of Scotland. 
Certain green schists which were evidently originally sediments, and 
probably tuffs, are associated with nimierous sills and bosses of eruptive 
material. The way in which these various rocks are grouped together 
strikingly suggests a series of volcanic products, some of the crushed bosses 
recalling the forms of true necks in younger formations. Eut they have 
been so enormously compressed and sheared that the very lavas which 
originally were massive amorphous crystalline rocks have passed into fissile 
hornblende-schists. 
Among the Pakcozoic systems of Britain, however, where considerable 
Fig. 81. — Diiigraiii illu.stratiiig the gradual emergence of buried volcanic coues througli the influence 
of prolonged denudation. 
fracture and displacement have taken place, examples of successive stages in 
the reappearance of buried volcanic cones and necks may be gathered in 
abundance. As an illustrative diagram of the process of revelation by the 
gradual denudation of an upheaved tract of country. Fig. 31 may he 
referred to (compare also Fig, 147). 
Here three volcanic vents are represented in different stages of re- 
emergence. In the first (A) we see a cone and funnel which, after having been 
buried under sedimentary deposits (s, s,) have been tilted up by subterranean 
movements. The overlying strata have been brought within the influence 
of denudation, and their exposed basset edges along the present surface of 
the land (//, (j) bear witness to the loss which they have suffered. Already, 
in the progress of degradation, a portion of the volcanic materials which, 
ejected from that vent, were interstra tided with the contemporaneous 
sediments of the surrounding sea-floor, has been exposed at t. A geologist 
coming to that volcanic intercalation would be sure that it pointed to the 
