C—2b

10

A consideration of the foregoing evidence leads me to believe that the coal-seams may reasonably be expected to extend for at least two miles (horizontal distance) beneath the valley. At great depths haulage and pumping costs will increase rapidly, and a point will be reached beyond which the coal cannot be profitably produced ; its position will fluctuate in accordance with market conditions, but it is unlikely that mining will reach a greater depth than 2,000 ft. The coal attains this depth at a horizontal distance of about 75 chains from the outcrop. I consider, therefore, that it is probable the seams continue beneath the plain beyond the limits of profitable mining. The behaviour of the coal-seams at greater depths is a matter of no economic importance. (b) The Nature of the Faulting which may be encountered in Dip Workings. In the extensive field of coal which extends beneath the Inangahua Valley the only faults which can be located are continuations of those which outcrop on the exposed portion of the field. No specific movements in the remainder of the field can be found by geological means. The rocks which form the toes of the spurs which project farthest into the alluvial flat are marine mudstones which cleave in places which appear at first glance to be true bedding-planes, but on close examination prove to be cracks formed by subaerial agencies, parallel to the contour of the surface. In the section of this report dealing with the structure of the field it is shown that major faults — i.e., dislocations—the throw of which is so great that they are economically impenetrable, are not likely to be encountered. It was shown that the faults are divisible into two classes : — (a) Downthrows : Usually monoclinal flexures. (b) Upthrows : Clean-cut faults. The former extend for many chains, and are of greater importance than the latter, which are seldom as much as 5 chains long. In the northern end of the field, however, upthrows are the predominant form of dislocation. The dip of a monoclinal flexure seldom reaches verticality, and as there is not often an actual rupture of strata an attempt should be made to " keep " the coal. The production of coal which lies at so steep an inclination will, naturally, not be directly remunerative, but unless the field has previously been bored in detail it is the safest policy. When the coal commences to flatten, a stone drive for haulage purposes can be immediately put in hand in the overlying strata, in a direction which has been computed from the positive data obtained in the small shaft. The extension of the main dip through strata over the coal-seam which has been thrown down by a monoclinal flexure may (as in the dip in Stony Batter Creek) cut one of the small upwarps which lie behind the main fold ; but, on the other hand, it might pass over the coal, and involve additional unnecessary expense in driving again at a steeper angle. When small upthrows are met (the following does not apply to the Capleston field, where the upthrows are often of considerable magnitude), which seldom extend for more than 5 chains, it is advisable to endeavour to drive round them in coal before commencing a horizontal stone drive—the throw of the fault will then be known, and the stone drive, if such be necessary, can be placed in the most advantageous position for haulage purposes. To sum up, in the southern part of the field the upthrow faults are clean cut, and have a small throw and prolongation, whilst the downthrow movements are folds which, extend for considerable distances, and have throws amounting in some cases to 100 ft. If any doubt exists as to which class a fault belongs the character of the neighbouring strata should be examined. It will often pay to excavate roof and floor for a few feet, in order that any necessary stone-driving may be intelligently prosecuted. In the lease held by the Burke's Creek Company the penetration of sandstone is indicative of a downthrow, whilst mudstone (usually with fine sandstone lenses) will be encountered at an upthrow fault when mining in No. 2 seam. On Morris's lease this applies only to a limited extent, for No. 2 seam is overlain immediately by several feet of mudstone with coal bands, which will be penetrated before the sandstone is reached. In the case of an upthrow on this lease, mudstone will be encountered as on the adjoining lease, but beneath it lies a sandstone similar to that above the seam. Hence a fault will probably be a downthrow if mudstone with numerous coal-streaks is found, and an upthrow in the case of mudstone which contains little or no trace of coal. When mining in other seams, or in other parts of the field, a glance at the map accompanying this report will show in a simplified form the general succession of strata in the neighbourhood of a seam. Stone drives can then be advanced accordingly. (c) The Flattening of the Measures in Depth. It is commonly supposed in Reefton that the inclination of the coal-seams will commence to decrease beneath the toes of the spurs which project into the alluvial plain. The assumption is founded on one or two coincidences. The height to which the alluvium rose up the former valley-slopes is a factor entirely independent of the structural movements which folded the strata. The beds are folded into a broad north-easterly-trending syncline. The average dips on the exposed portion of the coalfield must decrease gradually to zero at the base of the trough, and then increase progressively up the western limb of the syncline.