280 Mr. Hopkins’s Abstract ofhis Memoir on Physical Geology. 
mental in extending those existing previously. Partial eleva- 
tions, or subsidences, may be easily conceived to be thus pro- 
duced; but whatever alteration may take place in this man- 
ner, in the general conformation of the district, must be un- 
der the guidance, as it were, of the fissures previously ex- 
isting. 
Nothing perhaps will tend’'more to corroborate the views 
I have been explaining on this important point of the forma- 
tion of systems of fissures, than the attempts we may make 
to account for it otherwise, assuming always that the phe- 
nomena are due to the action of mechanical causes extraneous 
to the mass itself, and independent of that kind of internal 
molecular action to which the existence of joints, or of a la- 
minated structure, may possibly be owing. In the first place, 
I have shown that two parallel fissures not remote from each 
other could not be formed consecutively by a repetition of 
the elevatory action extending to the whole elevated mass; 
this consecutive formation, if it should take place at all, must 
therefore be owing to consecutive partial efforts of the eleva- 
tory force at different points of the mass. But I have shown* 
that, if the elevatory force be confined to a portion of the 
mass of comparatively small superficial extent, fissures must 
either be formed diverging from it in all directions, such as 
have been recognised in Mount Etna, and in the groups of 
the Cantal and Mont Dor, or concentric about the vertex, so 
that it is mechanically impossible that systems of parallel fis- 
sures could be thus produced. In fact, I can in no way con- 
ceive this successive formation of parallel fissures, without 
hypotheses respecting the mode of action of the elevatory 
force which are infinitely too arbitrary to be admitted for an 
instant. 
After one system of fissures is formed, there is no difficulty 
whatever in conceiving the formation of a second system per- 
pendicular to the former. The existence of two rectilinear 
parallel fissures must evidently destroy all tension in the por- 
tion of the mass between them, but will have no effect on the 
extension, or therefore on the tension which may exist in a 
direction parallel to the fissures, the only one in fact in which 
any tension can be impressed on a part of the mass so situ- 
ated. Consequently whatever tendency there may be to form 
a second system of fissures, it must necessarily be in a direc- 
tion perpendicular to that already existing. This second 
system might be formed by any forces, however partial or 
irregular their action, (always assuming ft not absolutely zm- 
* Memoir, p. 47. 
