S8 On the Constitution and Construction of the 



of the outer crust of the globe, some miles thick, which has 

 undergone an elevatory movement, and been fissured by it. 



R The cavity containing the fluid matter, whose expansion 

 or intumescence has heaved up A B. " The fissures," says 

 Mr Hopkins, " will scarcely ever be exactly parallel, and 

 therefore will meet if sufiiciently produced." The diagram 

 shews them before they have undergone any displacement. 

 Some of the separate masses are complete wedges, as b and h ; 

 some truncated wedges with their broad sides upwards, as df; 

 and some truncated wedges with their broad sides downwards, 

 SiS ace^i. The formation of these fissures will be completed 

 at nearly the same instant of time. Conceive the mass A B 

 to be then further uplifted. The fissures will not be farther 

 widened ; for the complete wedges b h, which do not reach 

 down to the fluid mass below, will descend by their gravity 

 into the position shewn in fig. 4. The truncated wedges also, 

 df, having their narrow sides downwards, will encounter lesS 

 resistance from the fluid mass below than ac e g i^ which have 

 their broad sides downwards, and will also descend ; and thus 

 the different masses will arrange themselves as shewn in fig 4, 

 forming an arch which will sustain itself. Supposing, then, 

 the cause of the intumescence of the fluid to cease acting, and 

 that fluid to return to its original dimensions, the pressure of 

 the superincumbent mass A B may thus be wholly or partially 

 removed from the fluid. " Hence, assuming that solidification 

 is promoted by great pressure, it evidently appears how a 

 portion of the interior mass might be maintained in a state of 

 fluidity by the removal of a superincumbent pressure, which 

 would otherwise have brought it to a state of solidity." 



" It is not essential to assume that the arch shall entirely 

 support itself. It may be partly supported by the fluid be- 

 neath, or it may break down in certain points, or along cer- 

 tain lines, and form there new supports intermediate to the 

 extreme ones. Instead of one continuous internal lake, a num- 

 ber may thus be formed, connected with each other by more 

 or less obstructed channels of communication." 



The phenomenon previously mentioned, which this hypothesis 

 so well explains, is the following : — When faults, or shiftings 

 of the strata, occur in mines, it is always found that the dis- 



