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534 GEOTECTONIC (STRUCTURAL) GEOLOGY. [Boox IV. 
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Fig. B). The upthrow or downcast side of the dislocation would be — 
determined by the observer’s knowledge of the order of superposition 
of the respective groups of strata. 
The existence of a fault having been thus proved from an 
examination of the geological structure of the ground, its line across 
the country may be approximately laid down—lIst, by getting ex-— 
posures of the two sets of rock, or the two ends of a severed outcrop — 
on either side, as near as possible to each other, and tracing the trend 
_of the dislocation between; 2nd, by noting lines of springs along the 
supposed course of the fault, subterranean water frequently finding 
its way to the surface along such fissures; 3rd, by attending to 
surface features, such as lines of hollow, or of ridge rising above 
hollow, the effect of a fault often being to bring rocks of unequal 
resistance together so as to allow the more durable to rise more or 
less steeply from the fracture.? 
Part VII.—ErvuptivE (Ienrous) Rocks As PART OF THE 
STRUCTURE OF THE HartTuH’s Onrust. 
The lithological differences of eruptive rocks having already been 
described in Book II. (p. 129), it is their larger features in the 
field that now require attention,—features which in some cases are 
readily explicable by the action of modern volcanoes; in other cases 
bring before us parts of the economy of volcanoes never observable 
in any recent cone; or reveal deep-seated rock-structures which lie 
far beneath the upper or volcanic zone of the terrestrial crust. A 
study of the igneous rocks of former ages as built up into the frame- 
work of the crust, serves to augment our knowledge of volcanic 
action. 
At the outset, it is evident that if eruptive rocks have been 

Fic. 266.—EXTENSIVELY-DENUDED VoLoANIc District (B.). 
extruded from below in all geological ages, and if at the same time 
denudation of the land has been continuously in progress, many 
masses of molten material poured out at the surface must have been 
removed. But the removal of these superficial sheets must neces- 
sarily have uncovered their roots or downward prolongations, and 
the greater the denudation the deeper down must have been the ~ 
original position of the rocks now exposed to daylight. In Fig. 266, 
for example, a section by De Ja Beche shows a district in which a 
series of tuffs and breccias (b b) traversed by dykes (a a) is covered 
unconformably by a newer series of deposits (dd). Properly to ap- 
preciate the relations and history of the rocks, we must bear in mind 
1 See “ Field Geology,” by the author, Chapter X. 
