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BEVIEWS. 
tance of 22 miles, and from near Llanrhaidr yn Mochnant, on the east, to the valley of 
Penmachno on the west, a distance of 24 miles; thus occupying an area of 400 or 500 
square miles at least. It probably was once much more extensive; because, though we 
reach its apparent original termination in one direction near Dinas Mowddwy, where it 
dwindles to a thickness of two or three feet, in others its present ‘outcrop’ shows no 
symptom of diminution of thickness or other sign of original termination.” 
On the subject of joints we must allow Mr. Jukes to speak for 
himself, premising that, in our opinion, the “ strike joints” of Sedgwick 
are really cleavage planes, and that the threefold principal joints in 
granite are due to crystalline, and not to polar or mechanical forces. 
We would except from this remark joint faces occurring in gneissose 
granite, as such faces are unquestionably due to mechanical pressure, 
and are a form of cleavage; they may he well studied in the granites 
of Donegal and Scotland :— 
“ For the production of natural blocks of rock there must clearly be, at least , two sets 
of joints in stratified, and three sets in unstratified rocks, each set more or less nearly at 
right angles to each other. 
“If we compare a set of stratified rocks to a pile of slices of bread, it is clear that to 
divide these into lumps, we must cut them in two ways, lengthwise and across. The 
unstratified rocks, however, would resemble the whole loaf, which we must cut at least 
in three directions in order to divide it into lumps, first horizontally into slices, and then 
lengthwise and across. 
“ In addition to these fewest possible sets of joints in the two kinds of rock, there are 
in reality others in various and irregular directions. 
“ If we pause here to inquire as to the general cause of joints, the only answer we 
can give is, that they are, in the first place, the natural result of the shrinkage or con¬ 
traction of rocks upon consolidation. 
“In examining the newly formed beds of stone in the small islands upon coral reefs, 
they are always found to be divided by joints like other rocks. The consolidation of this 
stone was obviously due to the action of rain-water dissolving part of the carbonate of 
lime, and redepositing it as a cement, so as to bind together the previously incoherent 
coral sand; for the stone generally rested on and was surrounded by coral sand still 
incoherent. Among the coral islands on the north-east coast of Australia I often ob¬ 
served several beds of stone resting on each other, each more than a foot thick, inclined 
at an angle of 8° or 10°; that is to say, at the same angle as the slope of the beach or 
bank of sand on which they rested. They had to all appearance been formed, that is, 
consolidated, in this position. The points which traversed them, although often uneven 
and jagged, ran in straight parallel lines over spaces sometimes of 200 yards, or as far 
as they could be seen, their planes being generally at right angles to those of the beds, 
one set of joints running along the greatest linear extension of the mass (‘ strike’joints), 
and the other set directly across the former, and in the same direction as the inclination 
of the mass ( k dip’ joints). 
“The directions of these two sets of joints seemed to depend in these cases on the 
directions of the principal bounding surfaces or edges of the mass. 
“ Professor Phillips tried many years ago, in his Geology of Yorkshire, whether the 
directions of the principal joints were not related in some way to the magnetic meridian, 
and arrived at results showing a tendency in the two principal sets of the joints of the 
Yorkshire rocks to arrange themselves according to certain magnetic bearings. This, 
however, seems to be only another way of stating that there are two principal sets of 
joints in the district, those of each set being parallel to each other. 
“ Professor Sedgwick refers the directions of joints chiefly to the lines of upheaval 
and disturbance in rocks, calling those which run along or parallel to the ‘ strike’ of the 
beds, ‘ strike joints,’ and those parallel to the ‘ dip,’ ‘ dip joints.’ All other joints he calls 
‘ diagonal’ joints. 
