78 Records of Lite Geological Survey of India. [vol. t. 
can be distributed into three groups. The numbers of observations in the two quadrants 
respectively do not at all represent the relative importance of the groups. The group C 
predominates greatly, dividing the whole rock-mass into cleanly separated vertical layers 
of from six inches to three and four feet in thickness. About an equal number of the cross¬ 
joints in the north-east quadrant were taken in order to obtain fair averages of their bearings 
among themselves. These cross-joints are much less regular than the main joints, often 
leaving unbroken spaces of thirty and forty feet: the planes, too, are less continuous, and 
frequently have a considerable underlie. 
It will be seen from the table that the three groups present a very satisfactory 
illustration of Professor Haughton’s explanation of joint-systems,* which assigns definite 
mechanical relations between the directions of the joint-planes traversing any mass of 
strata and those of the compressing forces that have affected them. The Primary system, 
generally the predominating one, following Sharpe’s law of cleavage, occurs at right angles 
to the main compressing force. The second system is determined at right angles to the 
Primary, and therefore at right angles to the direction of least compression, from which 
relation to the Primary it is called the Conjugate system. Other four systems, (respectively 
conjugate to each other) may be determined, related to the two main systems by the limiting 
angle of friction of the rock. There is no difficulty, for the case under notice, in assigning the 
direction of the compressing forces by which the rocks have been affected, and thus obtaining 
the key to*the induced rock-structure. Although, as a whole, these formations lie in their 
original position of deposition, there are many instances of even intense disturbance near the 
boundaries of the basin, where much of the compressing forces seems to have been expended, 
the yielding masses of the newer sedimentary deposits being here in contact with highly 
consolidated metamorphic rocks. Tbe flexures of the strata thus produced observe a very 
general parallelism to these boundaries. As already mentioned, the boundary upon which 
Jabalpur is situated has a very steady general bearing of about east-30°-north. In this 
immediate neighbourhood there is a single case of disturbance, and it conforms to the 
rule just indicated: at about half a mile to tbe south-east of the quarries there is a ridge 
formed of strata of the same formation dipping at 30° to south-3o°-east. The direction of 
the compressing forco being thus fixed, it is evident that tbe A group of joints, at right 
angles to that direction, represents very closely tbe Primary system; its mean bearing 
being 28°30' north of east. The group C, which is here the dominant one, then takes its 
place as the Conjugate system to A. Its mean range in the diagram is 23° 30' west of north, 
giving only 85° between it and A ; but this might evidently he brought much nearer to the 
normal by slightly limiting the range of the group to where the readings are concentrated. 
The group A would come in as a Secondary system to A 1 , the angle between them being 
22° 30'. Interpreted from this point of view, it would seem that the direction of the main 
joints C was determined at right angles to that of least compression. 
What it is chiefly desired to exhibit in this example is the conditions which seem to 
have produced so very local a peculiarity. It may, I think, he presumed that the power 
concerned was the shrinkage upon induration—a familiar agency in such phenomena; but it 
seems to have been seconded in an essential manner by local conditions, such as the texture, 
the homogeneity, the thickness, of these masses, or by their position upon a more rigid base. 
These determining secondary causes are, at least, suggested by the comparison of the 
Kattunga locality with others in the neighbourhood, where the induration is quite as great 
without similar jointing. Several such cases may he seen on the open ground to the east 
of the station, where there are a few scattered rocky hillocks, of about an acre in extent, 
formed of hard sandstone of the same age as the Ivattungd rock, but without regular 
jointing. 
* Phil. Trans, for 1858 and 1864. 
