MINING, 
125 
part. Contraction has severed the bonds of the continuity of 
the strata; and those fractures which are the result of the cool¬ 
ing process have been made in comparatively straight lines, or 
in given directions; or we may regard the causes of fractures 
simply as subterranean, but due to general conditions, and 
which must necessarily affect the whole of the cooling ^envelope. 
The fact that fissures may be, and probably are thus formed, 
is agreeable to all that is known of cooling bodies; and obser¬ 
vation which has been directed to those fissures proves, in the 
general at least, that the fissures are made in lines of bearing 
quite constant. We may not infer that a mechanical force is 
applied beneath a stratum, and has erupted those strata in the 
lines I have spoken, as a previous step in the formation of a 
vein. A cooling state has given rise to a state of tension, 
which increases in the direct ratio of the diminution of tempe¬ 
rature, which is finally too great to be borne, when the conti¬ 
nuity is broken. It is true that a subterranean force is often 
operative in the mode represented, and by which strata are 
uplifted and fractured; but it is more consonant to facts to sup¬ 
pose that vein fissures are the result of cooling and the great 
tension which arises therefrom. As a general rule the direc¬ 
tion which a fissure has taken was in a line of the weakest part 
of the stratum; but it is easy to conceive that a greater strain 
may be made upon a stronger part, so as to form a fracture in 
a line which is apparently along the strongest part of the 
stratum. In crystalline rocks the planes of lamination must be 
regarded as weaker planes, and hence it is that a very large 
proportion of our veins of magnetic iron lie along those planes. 
Having alluded very briefly to the force which has been 
operative in the formation of vein fissures, it is proper that I 
should speak of the manner and force by which they are filled. 
It is in the first place admitted that the matter which fills those 
fissures was liquid or semi-fluid at the time it passed into them; 
and furthermore, that the vent produced by over tension ex¬ 
tended to the liquid or semi-fluid mass below. It is inconsist¬ 
ent with known facts to suppose the fissures to have been filled 
