TIIE STRUCTURE OF ROCK MASSES. 
117 
land ; when a force operates in elevating or depressing any 
large and rigid mass of rock, it must necessarily tend to de- 
velop a series of fissures or cracks, more or less regular in 
proportion as the force itself has acted with more or less 
uniform intensity over the area in question. 
In genera], the parallelism of joints is somewhat deceptive, 
as the lines of fracture are frequently in reality only radii, yet 
at the same time, this divergence may be but little observable, 
owing to the great length and number of the radiating fractures. 
Although such joints may occasionally possess a considerable 
degree of regularity or parallelism over a somewhat extensive 
area, still this is rather the exception than the rule, as it is much 
more usual to see them developed in several directions at the 
same time, and at the most discordant angles to one another, so 
that the rock mass may be thus cut up into blocks more or less 
symmetrical, owing to the crossing of the different systems of 
joints, which are not necessarily of the same geological age. 
The second class of joints is confined entirely to eruptive 
rocks, being due to the contraction experienced by such rocks 
when cooling : these can commonly be seen well developed in 
most quarries of hard stone, such as granite ; here it is found 
that the direction of the joints follows in great measure, or is 
parallel to, the external configuration of the rock mass, which 
causes the formation of a series of more or less thick layers of 
separation (or benches, as they are frequently called), in some 
cases resembling true stratification, and not unfrequently 
thought by geologists, not well up in the study of such crystal- 
line rocks, to be so many proofs of the sedimentary origin of 
such rocks. 
As the cooling of such a mass of eruptive rock proceeds from 
the exterior inwards, layer after layer of the consolidated rock 
separates itself from the main mass below it by the effects of 
contraction, and from the same cause another set of vertical 
joints becomes developed in these layers at right angles to the 
first series, which tend to break them up, in turn, into more or 
less regular blocks. 
In some of the more fine-grained volcanic lavas, basalts, 
phonolites, &c., similar parallel jointing may occasionally be 
found present, so numerous and close to one another as to give 
quite the appearance of lamination, and in some cases even to 
permit of the rock being split along them into tolerably thin 
plates. 
Considered from an economic point of view, both these classes 
of joints are of the greatest importance ; for it is only by taking 
advantage of their occurrence, that many massive rocks, such as 
granite, syenite, porphyrite, &c., which otherwise possess no 
divisional planes, can be quarried with facility. In mining ope- 
