Transactions of the Geological Society, 1st series, vol. 4/On a shifted Vein occurring in Limestone
XIX. On a Shifted Vein occurring in Limestone.
By the J. Mac Culloch, M.D. F.L.S. President of the Geological Society, Chemist to the Ordnance, Lecturer on Chemistry at the Royal Military Academy, and Geologist to the Trigonometrical Survey.
[Read November 15th, 1812.]
The shifting of veins on a small scale is much more common
than the larger phenomena of this nature, which are nevertheless
of frequent occurrence wherever veins exist. These larger
dislocations are known to arise either from the lateral motion or subsidence
of the containing parts, the sides of the line of fracture
sometimes remaining in contact, and being at others separated by
a vein of another description, too well known among miners to
need any comment here. I know not why mineralogists have sometimes
imagined that such appearances were in the smaller examples
fallacious, and were contemporaneous with the formation of the
containing stone. The vein represented in the accompanying drawings
is at any rate too remarkable to admit of such an explanation,
and its character is sufficiently decided to establish a general rule
in favour of all similar appearances.
The rock in which this shifted vein is contained is a secondary
limestone, and was brought from Ireland. The specimen, which
formed a mill-stone of six feet in diameter, belongs to the Royal
Powder Works at Waltham Abbey.
The inspection of the drawing will show more readily than any description, that the vein consists of a series of separate fragments, having somewhat of a general parallelism, with a correspondence at any two neighbouring extremities, such, as to render it a matter past doubt that they have once formed a continuous line.[1] To displace such a vein into its present position must have required a series of slides or shifts, each advancing by nearly a similar space beyond the one preceding it. But this sort of echellon movement will be very visible in an outline or diagram in which I have attempted to replace the vein in its original position, and also to trace the alignement on which each part must have moved to its present place.
The deficiency of parallelism occurring in this diagram arises from my having intentionally left the replaced ends at a small distance, that their correspondence might be more visible.
There is now no appearance of slide or fissure, or discontinuity
of any sort in the mass, but the texture of the whole is uniform
and continuous. As the specimen has been completely and highly
polished, there can be no doubt respecting the accuracy of this
observation. It may afford matter for speculation to inquire in
what condition the rock must have been to have undergone this
change. It has probably consisted originally of a series of thin strata,
which having been at some subsequent period fissured at an angle,
have admitted the infiltration of the white carbonat of lime which
now constitutes the vein in question. That it was perfectly hard
at the time of this change the angularity of the fragments shows.
The same solution which filled the vein has probably joined the
laminæ, and cemented the whole once more into a solid mass, although
the junctions are no longer visible.
What the situation of the present vein is with respect to the horizon I have no means of knowing, nor is it a matter of much moment, as it is tolerably evident that the strata which are now visible on the surface of the earth do not always lie in the positions in which they were formed. There can be no question that slides of this nature are of different æras, as we may often observe a succession of them in which the first has been shifted by a subsequent one. Whether those represented in the present drawings are referable to one or to different periods, there is no appearance to decide, though their uniformity would lead us to suppose that they were all produced by a single cause and at one time.
Some unnecessary doubts appear to have been entertained by geologists
respecting the formation of those smaller veins which have their
origin and termination in the rock where they are found, and which
have no communication from without. While one party has denied
their posterior origin to the rock in which they are contained,
and asserted that they were of “ contemporaneous formation” with
the containing parts, another has had recourse to an igneous hypothesis
for the purpose of solving a difficulty of which the explanation
appears abundantly simple. It is universally known that many
rocks contain much water in a state of intimate mixture, or perhaps
combination, which they are subject to lose on drying or by exposure
to the air. From this cause they contract and form fissures.
Similar fissures occur from the ordinary subsidences and fractures
of parts either ill supported or subjected to external violence. Such
cavities being formed the process of infiltration commences. The
water existing in the rock percolates into the cavities, sometimes
forming crystals, and sometimes filling the cavity with a solid mass
of the matter which it held in solution. When silica exists in the
rock, veins of quartz are thus formed; when lime, calcareous spar.
The solution of these earths in water is unquestionably more perfect in nature and the solutions more saturated than those which we can produce in our laboratories. Doubtless there is a state of division which renders them thus easily soluble, and which is perfectly analogous to that state which these earths, and notoriously silica, are subject to even in our little experiments. The gradual formation of quartz veins is too slow perhaps to be witnessed, but it may be conjectured from the various states in which they are seen, sometimes forming a detached and distinct crystallization, at others a solid mass, and visible more particularly in the schistose rocks. In limestone, the progress being more rapid, is more obvious. In the marble beds of Glen Tilt admirable examples of this process are to be seen. Fissures are here of common occurrence in the exposed layers. If we examine these, the thinner parts are found filled with a solid mass of crystallized carbonat of lime. Towards the center, where a the fissure is wider, crystals are seen approaching into contact, while further on, the walls of the crack are lined with the first efflorescence of carbonat of lime, an efflorescence destined at no long period to cement and reunite the whole. Water charged with carbonat of lime is also found in the cavities when a successful fracture of them can be obtained.
This then is the secretion by which these veins are filled up, and it offers a demonstration of which the several steps are as perfect as if we actually saw them succeeding each other. There is no reason to doubt that the stalactitical chalcedonies of the trap rocks are produced in a similar manner, and that many at least of the onyx pebbles owe their origin to a similar cause. There is equally little difficulty in explaining by the same process the supposed obscure septaria; where the contraction of the softer parts of the compound mass has left cavities defining those obscurely columnar forms which clay acquires on drying, and where the calcareous earth, resident in the mass itself, or in the surrounding beds, has been gradually brought into solution by water, and deposited wherever it could find a cavity in which to crystallize.
In the diagrams Nos. 3 & 4,[2] I have supposed a section of the rock containing the vein, for the purpose of exhibiting the number and extent of the slides which must have occurred to produce its present position. Where similar appearances are observed on a large scale, it will be apparent how much the form of the containing rock must change to admit of the motion of the included vein, and how the subsidence of a mountain must have followed the sliding of a large vein; from which slide also the quantity of subsidence may be easily estimated.