SCIENTIFIC TALKS

Otago Witness, Issue 1905, 14 August 1890, Page 41

SCIENTIFIC TALKS

By N. D. 0.

BOCK JOINTS AND VEINS«

We have seen how joints may be caused by the same agencies which have brought about a metamorphosis in the rocks themselves. Two of these causes, pressure and heat, are almost counterparts of each ether in their action, for the heat causes pressure by expanding the jock, but with thiß difference, that there is likely to be a greater amount of contractidn on cooling. In addition to movement, it has been advanced that jointing may b6 duo f o crystalline force or terrestrial magnetism.

While no one, cause can be put forward as the only or even the predominant one, experiments have clearly shown that a set of joints oan be formed by each of the first mentioned ; but how far magnetism can be regaided as producing or aiding such a result is most difficult to determine, although the constancy of the magnetic meridian in New Zealand to the austral pole, unlike the variation in the northern hemisphere, would lead one to expect a uniformity in the bearings of the joints, if this were at all an important factor, which has not hitherto been observed.

, Magnetism has also been considered an influence in the distribution of metals in their lodes, a theory which is on the face of it more probable. Again, there are ordinary surface deposits such as brick clays, which do not seem to have been subjected in an appreciable degree to any of the firstmentioned causes, and yet are freely intersected by joints, very variable, perhaps, but not more so than might be expected from the softness of the deposit, and occasionally running for considerable depths and distances.

The same agencies have brought about that re-arrangement in the rocks which is known as cleavage, and which is distinct from our subject in the regularity of the planes and in the alteration of the rock particles, so that tbe longer axes are parallel to these lines, and also in the development of new minerals, more especially mica. This particular texture is usual only in the finest clays, and its typical example is roofing slate.

The economio aspect of the part that rock joints play in Nature is by no means an unimportant one. 'In the stratified rocks their practical value ia seen in quarrying, where they enable large blocks of stone to be obtained, and perhaps even more particularly in the working of coal, in which the workings run at right angles on the joints ; at that set which is more clearly defined and at which more coal can be won, the miners term the workings as on the "plane,' while those places which branch off to right and left are said to be working on the "end." This utracture is less marked in our lignites than in most true coals, which have a cuboidal fracture, whereas in the brown coals it is frequently sub-conchoid al, although in some cases the latter are very freely jointed. In the igneous rocks the jointed structure is neither so apparent nor so regular, probably from the absence of the planes of stratification as a base to which the joints can range themselves more or less at right angles. In such compact rocks as basalt they aid the quarryman, although at times passing into veins filled with softer material , detrimental ts the value of the stone. Sir J. Hector in the geological report for 1885 drew attention to the interesting fact that the Kaitangata (conglomerates are cut by remarkable oracks filled with thin veins of cale-spar. These intersect both the pebbles and cement without causing faulting. He mentioned that they are possibly caused by sharp earthquake shocks and states that they are quite distinct from the great faults which traverse the coal measures. It is not quite clear on what the argument for this is based, but as far as can be judged from the short compass of the notice, it is founded on the cracks extending into the newer conglomerates and the improbability of the faults likewise displacing these. Should it be proved, however, that the conglomerates are all of one system— and this is the whole tr«nd of recent geological research — it would become necessary to uphold this view to suppose that the faulting took place during cretaceo-tertiary time and prior to the deposition of the upper beds; but as this would entail at least a local unconformity, the " omniverousness," as Professor Hutton terms it, of the cretaceo-tertiary system is likely to be here Maintained. No doubt the cale-spar has been deposited by water, which circulates in these joints and faults ; and that it is not confined to merely percolating through the interstices of the rocks, but flows through the underground channels which theße afford it, is proved by the fact that live fish have risen to the surface from the artesian wells of the Sahara ; and although this may be exceptional, and might be ascribed to the opening boine: the outlet of some subterranean lake, what we are contending for is confirmed by the rise of leaves and even twigs- of trees in other artesian waters. In the same way water has been a prime factor in the infilling of the larger veins, and probably also of their contained metals, though the latter is not soeasy of demonstration.

Perhaps nowhere have the rock joints a wider influence than in the outlining of the' scenery. To them are due the square masses in granite resembling stratification, which have become classic aa Cyclopean walls or masonry, and in basalt they are the cause of the beautiful columnar structure which has made the Giant's Causeway and Fingal's Cave famous, and instances of which are to be seen around Dunedin. Joints we have seen form natural water channels, and as these come within the reach of frost a wedge, or more properlj a block of ice, is forired which has a wedge-like astiou in forcing off those blocks which are less secure along the sides of cliffs, and this fact is one of the causes of their steepness, ibe frost action in such rocks exceeding that of weathering. As well as forming lines of weakness for tbe action of frobt, they are attacked also by running water, more especially in the case of the recession of a waterfall, such as Niagara, where jointed sandstone rests on soft shales, which, as they get undermined by the water, leave the overlying jointed rook unsupported, to toll into and be swept away by the boiling

torrent. And it has been pointed out that if this cutting out of the gorge is not interrupted by a change in the strata, or in their dip, placing the shales beyond the reach of the water, eventually Lake Brie itself will be drained by this means. While the above gives one of the most striking examples of the influence of jointed structure on rock erosion, the very much larger scale on which a similar action takes place is exemplified in an extensive high coast line, where although the rate of recession of the cliffs is slow and hindered by the talus slopes which form at their base, yet these are attacked by the breakers and their batteries of boulders continually being hurled against the oliffa, so that the denudation in the aggregate is immeasurably more.