M. HOPKINS, ON RESEARCHES IN PHYSICAL GEOLOGY. 



43 



Hence then we may conclude that the extension of the physical hne 

 ^C^ under the circumstances supposed, will be at least equal and 

 generally greater, than that of any similar line in the higher .ttio,^ 

 of the uphfted mass It seems also probable, that in cafes oH™: 

 :n nature the ea,ens.l^kty will be less in the lower portion of the de 



Now the tendency of any horizontal portion of the mass to separate 

 so as to form a vertical fissure, will vary directly as the .......r and 



inversely as the e.te.sMHy. We may therefore^afely crZe that 

 when a mass has been elevated as above supposed, the Latest tendencv 

 to rupture will not be in its upper portion and cons^n y that if 

 ^.y fissure be produced, whether by a gradual increase of the L ont 

 tension or by any more sudden impulsive action on the mass n 



w i:r;^^:ir- -^^ -^ — - - -^z- ^--::: 



37^ It appears, from what has been proved in the previous Section 

 that If we suppose the fissure produced solely by the tLsions to w ich 

 the mass IS subjected, the plane in which it will lie will be perpendic^kr 

 to the dn-ection of the single system of tensions which, n 'd s e / 

 act upon the mass, and will consequently decline as much f om a 

 vertical plane as that direction deviates from horizontality. Ac ording 

 to the hypothesis we have made, however, of the force acting onhf 

 elevated mass through the medium of an elastic vapour thif "Jur 

 will necessarily ascend into the fissure, and exert a 'fluid pr ssureTn 

 ;ts sides, m a direction perpendicular to them, and of whfch the in 

 tensity may bear a considerable ratio to that of the tension. To fo m 

 a rough estimate of this intensity, let . be the radius of the dr ^ 

 which shall most nearly coincide with the curve ACB (Fi. p 41 t 

 he pressure of the fluid on a unit of surface, T the intensity o^'th^ 

 tension (supposed uniform) of the elevated mass estimated as in the 

 previous section, and b the thickness of the mass. Then the who e 

 tension exerted on a portion of the mass included between twve 

 planes perpendicular to the axis of elevation nf „ ^- . vertical 



each Cher, wU. . ,T. and „e sh.„ tlZ\L """" '"•"' 



f2 



