Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/352

 ting. The greater part of each vein is " dead " or unproductive ; the productive portions or " pockets " vary from a few yards to 100 fathoms in length and from a few inches to nearly 30 feet in thickness. The pockets are sometimes very close together, sometimes more than a mile apart in the same vein ; in all cases they are connected by a vein-" track," consisting chiefly of softened clay-slate and quartz with occasionally a little iron-ore (fig. 2).

The " pockets " are found not to descend parallel to the line of their dip, but to slope endwise, generally to the west, but in one or two cases to the east. To this phenomenon the author gives the name of " end-slant." He accounts for it in the following manner. He

Fig. 2. — Longitudinal Section of a Vein.

assumes that the veins have been segregated from the adjoining clay- slate, the unproductive portions occurring where the continuous strata were not sufficiently impregnated with ferruginous matter to produce a lode of ore, the " end-slant" of each productive part being " determined by the line of intersection of the sloping plane of the vein with the boundaries of the ferruginous portions at the commencement and termination of each 'pocket.' "

Discussion.

Mr. Etheridge thought that the great iron-lodes of this district lay in the great faults which traverse the country, and in which there had been considerable downthrow to the north. In most cases in the Bristol district the lodes seem to have been formed at the bottom of the sea during the New-Red-Sandstone period by infiltration of salts of iron into the faults.

3. On the Salt-mines of St. Domingo. By F. Ruschhaupt.

(Communicated by Sir R. I. Murchison, Bart., K.C.B,, F.R.S., V.P.G.S.)

[Abridged.]

The author described the Cerro de Sal, or Salt-mountain of St. Domingo*, as subordinate to the main chain of mountains running in a S.E. and N.W. direction, its own direction being E.S.E. and W.N.W. The eastern part of the Cerro de Sal is very rugged and steep, rising to 550 or 600 feet; towards the W. it becomes lower, and forms a chain of irregular hills.


 * See Quart. Journ. Geol. Soc. vol. xxiv. p. 335.