SYMMETRICAL STRUCTURES OF ROCKS. 91 
symmetry of structure, and impress the mind more firmly with a notion 
of some kind of crystallization. This impression hardly occurs at all to 
the observer of the clays of the oolitic and other more recent systems of 
the rocks. Similar remarks apply to the calcareous and arenaceous por- 
tions. All this seems in favour of the opinion that the consolidation 
and structural changes are due rather to length of time than to any 
peculiarity of agency. But there are facts to render this inference 
extremely dubious ; especially this, that the consolidation of the grau- 
wacke rocks must have been completed before the formation of the 
old red sandstone conglomerate of Cumbria, — and that of the mountain 
limestone before the age of the Brockram beds of Stenkrith. May we 
then venture to consider the degree of consolidation as depending on 
the degree of heat applied, as well as on pressure and the duration 
of its influence ? This question will be better answered after we have 
considered the structural peculiarities of the rocks. 
Besides the stratification or bedding of rocks, and the parallel or 
oblique lamination, which may often be noticed in their beds, both 
which are original or formative structures, there is another pervading 
alike stratified and unstratified rocks, viz., the divisional structure. 
All the apparent divisional planes of rocks, which are not coinci- 
dent with surfaces of stratification, or laminae of deposition, may in 
general be called joints, but it is convenient to adopt the term crack 
for such as are confined to only one bed of stone, joints for such 
as pass through one or more similar beds, and jissures for those greater 
divisions which dissect whole formations. ( See Guide to Geologij.) 
The most remarkable of these latter may be called master-fissures, 
lhe non-apparent, however real, fissility of rocks, exemplified specially 
in slate, is called cleavage. We shall first treat of joints. 
Joints in relation to the nature of rocks All rocks are traversed by 
joints, because concentration from expansion (whether aqueous or igneous) 
has happened to all rocks; — but in rocks of different chemical and 
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