204 Br. Richetts — On Fissures, Faults, Contortions, etc. 



also with tlie general sequence given of volcanic phenomena, which 

 more frequently commence with an explosion of steam and gases, 

 carrying upwards immense quantities of scoria3 and ashes, and not 

 unfrequently blowing off the summits of cones formed by previous 

 eruptions. Such explosions may be compai-ed to those induced by 

 water obtaining access to metal molten in a crucible, and the effects 

 are verj^ similar. This state, after continuing for a time, is followed 

 by streams of lava, which "well out, sometimes almost with the 

 tranquillity of a water spring, from the same or contiguous openings." 



Most remarkable has been the intrusion of lava between beds. 

 However powerful the force which has produced this effect, there is 

 in innumerable instances no indication of violence. Belts of Green- 

 stone and Basalt have been frequently insinuated between the strata, 

 so exactly in the direction of the bedding, that were those situated 

 above not altered by heat as well as those below, it would be with 

 great difficulty determined that they were intruded, and not inter- 

 stratified. 



There is much proof of a local connexion between volcanos and 

 earthquakes ; the former being generally preceded and accompanied 

 by the latter. Earthquake-shocks are far from being confined to the 

 neighbourhood of volcanos.^ They appear to be prevalent in locali- 

 ties which there is reason to conclude are areas of elevation or else 

 of depression. In endeavouring to account for their production, 

 those who have paid most attention to the phenomena accompanying 

 them appear entirely to overlook the geological changes produced 

 by faults, each of which must be a register of at least one, and most, 

 in all probability, record a succession of earthquake-shocks. 



Previous to a fault being formed, a fissure must have taken place 

 in the crust of the earth, and have progressed so as to cause a greater 

 or less interval between the sides of the fissure. This will occur 

 either upon depression or elevation of the surface, increasing as the 

 depression or elevation becomes greater. Should there be a stratum 

 so far unconsolidated that (as the rocks have lost their lateral sup- 

 port by the separation of the sides of the fissure), it cannot support 

 the pressure of the overlying beds, the result must be that wedge- 

 shaped masses will separate and fall or slip downwards, in a 

 manner similar to what occurs to a block of masonry, however solid, 

 if its foundations are undermined ; or in the same manner as that of 

 landslips along our coasts, only that in faults the wedge-like mass 

 is checked when it comes in contact with the opposite side of the 

 fissure, causing that thud or blow which occasions the vibration of 

 the earth's surface, and constitutes the earthquake. 



Few, if any, will now believe that any great faults are the result 

 of one single movement, but that they are rather to be attributed to 

 a long succession of displacements ; and it is likely that some do 

 not, through their whole extent, belong to the same geological era. 

 Faults not unfrequently have caused enormous dislocations of the 

 strata ; in North Wales to the extent " of 2,000, 5,000, or even 

 12,000 feet." Immense wedge-shaped masses, such as in these cases 

 ^ See Scrope, " Volcanos," p. 8. 



