116 
POPULAR SCIENCE REVIEW. 
slate, on the occurrence of coloured zones or bands of dif- 
ferently tinted stone. 
Although, as before mentioned, all sedimentary strata have 
originally been deposited in horizontal or nearly horizontal 
beds, this uniformity of arrangement has, in the majority of 
eases, soon been broken up by dislocations, depressions, or ele- 
vations, caused by various disturbing influences ; it is, however, 
beyond the scope of the present communication, to enter more 
i-uto detail as regards abnormal stratification, since the present 
remarks are more exclusively directed to the consideration of 
stratification as a parallel structure only. 
Joints . — In all the rocks, no matter of what geological age, 
or whether they are stratified or unstratified, there will be 
observed numerous lines of fracture, cutting through the rock 
mass in all directions, and at angles differing more or less from 
one another. Such divisional planes cannot, as a rule, be said 
to be parallel to one another, yet it is, at the same time, so often 
found that natural joints traverse even considerable areas as a 
series of straight and well-defined parallel lines, that it becomes 
imperative that this structure should be here taken into con- 
sideration, to avoid confounding such jointing with the parallel- 
ism of either stratification or cleavage, for numerous examples 
might be cited in which able geologists have even in the present 
day fallen into such errors. 
In distinguishing parallel joints from all other varieties of 
parallel rock structure, it is most essential to remember that 
joints either are, or at one time must have been, mere true 
fractures or fissures ; these cracks may at the present time still 
remain open, but they also may have become cemented together 
again, by infiltration or other causes, in some instances so 
firmly that the rock may now appear equally as solid as it was 
originally before the formation of such fissures ; in which event, 
however, the difference in appearance of the cementing material 
usually indicates where they had originally been. 
As joints, therefore, are mere lines of fracture, it naturally 
follows that the mass of rock enclosed between two such joints 
cannot, as is the case in both stratification and cleavage, possess 
in itself any structure enabling it to split up parallel to the sides 
of the joints themselves, yet in some cases there may still remain 
a doubt as to whether such a structure belongs to the class of 
joints, bedding, or cleavage. A somewhat more extended ex- 
amination of the neighbouring rocks will generally solve the 
question ; the parallelism of joints, however, is seldom or ever 
found to be more than a mere local occurrence. 
Joints may be divided into two classes : first, those which 
owe their origin to purely mechanical agency, as in the case of 
those accompanying the dislocation, elevation, or depression of 
