Dr, Bicketts — On Fissures, Faults, Contortions, etc. 203 



caused progressive subsidence, and so have formed bays and to some 

 extent deltas at and near their mouths. 



As a consequence of subsidence in one place, there must of 

 necessity be elevation in another. Elevation would also result from 

 the removal of pressure. The subject was then alluded to only for 

 the purpose of accounting for the partial emergence of the land, 

 upon the melting of the snow which covered its surface during the 

 Glacial period ; but it continually happens that those strata which 

 are physically highest above the sea-level, are geologically the 

 lowest in the series, the greatest amount of upheaval occurring 

 where denudation has taken place to the greatest extent. 



These innumerable evidences of changes of level occur in all parts 

 of the globe; they date from the earliest period registered in the 

 geological record, and extend down to the present time. Their 

 occurrence cannot be so general, or their progress so continuous and 

 gradual, if the interior of the earth is not to a certain extent in a 

 state of fluidity, and covered with what, compared with the whole 

 mass, must be a thin crust. 



It is very generally conceded that " the effect of elevatory force 

 by raising the mass under which it acts, would place the strata in a 

 state of extension, and therefore of tension," which continuing will 

 produce fissures even so as to extend through the whole thickness of 

 the earth's crust. To fractures thus induced the volcanos of the 

 Andes may be attributed. But a similar result has not so frequently 

 been ascribed to the effect of subsidence, though tension and exten- 

 sion, and therefore fissures, must occur under such circumstances, 

 commencing from a depth coinciding with that of the chord of the 

 arc of the earth's circumference included within the subsiding area. 

 That such fissures do occur in areas which are those of depression is 

 proved by volcanos being frequently situated in localities known to 

 be subsiding, and the presence of Trappean rocks is very frequent in 

 the Palgeozoic formations of Britain interstratified with sedimentary 

 deposits ; they were at their respective periods erupted, and poured 

 forth either as volcanic ash or lava-flows in areas which were not 

 only subaqueous but also subsiding, as is proved by the large 

 amount of subsequent accumulations. 



An examination of Trappean dykes tends to prove that the molten 

 rock " has been forced up through previously formed fissures to a 

 certain height," in its passage baking but not disturbing the rocks 

 which form the walls of the fissures. Lava can only proceed so far 

 as the fracture extends, and, as in subsiding areas there must be 

 compression of the upper strata, it can only make its way iipwards, 

 as such, so far as the chord of the arc before referred to. The only 

 manner by which I can suppose the vent to be formed in such a 

 case is by water, on obtaining access at "a greater or less depth to the 

 incandescent lava contained in the fissures, exploding, and carrying 

 away the superficial strata ; for it frequently happens that in what 

 in all probability are the pipes of volcanos passing through con- 

 temporaneous strata, there is no appearance of fracture or fault ex- 

 tending to a distance into the sedimentary rocks. This coincides 



