[105]

Fig. l. represents the case of a valley of Denudation in stratified rocks, terminated abruptly by a cliff on [106] the sea_shore; this figure is intended to illustrate two causes of the production of Springs by descent of water from porous strata at higher levels; the first, producing discharges in vallies of Denudation, along the line of junction of porous with impermeable strata; the other, by the interruption offered to descent of water by Faults that intersect the strata.

The Hills A, C, are supposed to be formed of a permeable stratum a, a', a", resting on an impermeable bed of Clay b, b', b''. Between these two Hills is a Valley of Denudation, B. Towards the head of this Valley the junction of the permeable stratum a, a', with the Clay bed b, b', produces a spring at the point S.; here the intersection of these strata by the denudation of the valley affords a perennial issue to the Rain water, which falls upon the adjacent upland plain, and percolating downwards to the bottom of the porous stratum a, a', accumulates therein until it is discharged by numerous springs, in positions similar to S, near the head and along the sides of the vallies which intersect the junction of the stratum a, a', with the stratum b,b'. See V. I. p. 559*

The Hill C, represents the case of a spring produced by a Fault, H. The Rain that falls upon this Hill between H, and D, descends through the porous stratum a", to the subjacent bed of Clay b".

* The term Combe, so common in the names of upland Villages, is usually applied to that unwatered portion of a valley, which forms its continuation beyond, and above the most elevated spring that issues into it; at this point, or spring head, the valley ends, and the Combe begins. The conveniences of water and shelter which these springheads afford, have usually fixed the Site of the highest villages that are planted around the margin of elevated plains.

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The inclination of this bed directs its course to wards the Fault H, where its progress is intercepted by the dislocated edge of the Clay bed b' and a spring is formed at the point f. Springs originating in causes of this kind are of very frequent occur rence, and are easily recognized in cliffs upon the sea~shore.* In inland districts, the fractures which cause these springs are usually less apparent, and the issues of water often give to the Geologist notice of Faults, of which the form of the surface affords no visible indication. See V. I. p. 560, Note.

Fig. 2. Section of the valley of Pyrmont in Westphalia. A cold chalybeate water rises in this valley at d. through broken fragments of New Red Sandstone, filling a fracture which forms the Axis of Elevation of the valley. The strata are elevated unequally on opposite sides of this fracture. See V. I. p. 561. (Hoffmann.)

Fig. 3. Section reduced from Thomas's survey of the mining district of Cornwall (1819); it exhibits [108]

* Three such cases may be seen on the banks of the Severn near Bristol, in small faults that traverse the low cliff of Red Marl and Lias on the N. E. of the Aust Passage. See Geol. Trans. N. S. Vol. l. Pt. 11. Pl. 37.

the manner in which the Granite and Slate near Redruth are intersected by metalliferous Veins, terminated abruptly at the surface, and descending to an unknown depth; these Veins are usually most productive near the junction of the Granite with the Slate, and where one Vein intersects another. The mean direction of the greatest number of them is nearly from E. N.E. to W. S.W. They are inter-sected nearly at right angles by other and less nmerous Veins called Cross Courses, the contents of which usually differ from those of the E. and W. veins, and are seldom metalliferous.

The Granite and Killas and other rocks which intersect them, e. g. Dykes and intruded masses of more recent Granite, and of various kinds of porphyritic rocks called Elvans (see Pl. 1, a 9. b. c.) are considered to have occupied their present relative positions, before the origin of the fissures, which form the metalliferous Veins, that intersect them all. (See V. I. p. 550.)*

* In Vol. l. p. 652, Note, a reference is made to some important observations by Mr. R. W. Fox on the Electro-magnetic actions which are now going on in the mines of Cornwall, as being likely to throw important light on the manner in which the ores have been introduced to metallic veins.

The following observations by the same gentleman in a recent communication to the Geological Society of London, (April, 1836,) appear to contain the rudiments of a Theory, which, when maturely developed, promises to offer a solution of this difficult and complex Problem.

"If it be admitted that fissures may have been produced by changes in the temperature of the earth, there can be little difficulty in also admitting that electricity may have powerfully influenced the existing arrangement of the contents of mineral veins. How are we other wise to account for the relative position of veins of different kinds with respect to each other, and likewise of their contents in reference to the rocks which they traverse, and many other phenomena [109] observable in them? Copper, Tin, Iron, and Zinc, in combination with the sulphuric and muriatic acids, being very soluble in water, are, in this state, capable of conducting voltaic electricity; so, if by means of infiltration, or any other process, we suppose the water to have been impregnated with any of these metallic salts, the rocks containing different salts would undoubtedly become in different or opposite electrical conditions; and hence, if there were no other cause, electric currents would be generated, and be readily transmitted through the fissures containing water with salts in solution; and decompositions of the salts and a transference of their elements, in some cases, to great distances, would be the natural result. But, on the known principles of Electro-magnetism, it is evident that such currents would be more or less influenced in their direction and intensity by the magnetism of the earth. They cannot, for instance, pass from N. to S. or from S. to N. so easily as from E. to W. but more so than from W. to E. The terrestrial magnetism would therefore tend, in a greater or less degree, to direct the voltaic currents through those fissures which might approximate to an east and west bearing, and in separating the saline constituents, would deposit the metal within or near the electro-negative rock, and the acid would be determined towards the electro-positive rock, and probably enter new combinations. Or, the sulphuric acid might, by means of the same agency, be resolved into its elements; in which case the sulphur would take the direction of the metal, and the oxygen of the acid, and in this way, the metallic sulphurets may have probably their origin; for, if I mistake not, the metallic sulphates, supposing them to have been the prevailing salts, as at present, would be fully adequate to supply all the sulphur required by the same metals to form sulphurets; indeed more than sufficient, if we deduct the oxide of tin, and other metalliferous oxydes found in our mines. The continued circulation of the waters would, in time, bring most of the soluble salts under the influence of these currents, till the metals were in great measure separated from their solvents, [110] and deposited in the East and West veins, and near the rocks to which they were determined by the electric currents."

In a Letter to the Author upon this subject (June 29, 1836), Mr. Fox further remarks:

" It should be observed that in proportion as the deposition of the metals proceeded, the voltaic action must necessarily have been considerably augmented, so as to render it highly probable that the metals were chiefly deposited at rather an early period in the history of the containing veins; and their intersection by other veins seems to strengthen this probability."

Mr. Fox has found by experiment that when a solution of muriate of Tin is placed in the voltaic current, a portion of the metal is determined towards the negative pole, whilst another portion in the state of an oxide passes to the positive pole. This fact appears to him to afford a striking illustration of the manner in which Tin and Copper have been separated from each other in the same vein, or in contiguous veins, whilst these metals also very commonly occur together in the same vein.