On some boulders of pseudo-jasper found near Newton Abbot (1910)
|Author(s):||Lowe. Harford J.||Origin:||DA Transactions|
By Harford J. Lowe, F.G.S.
(Read at Cullompton, 28th July, 1910.)
Introduction. The rock masses which form the subject of these remarks are met with in various places, occasionally as isolated blocks on the wayside, but are most in evidence at Whiteway Barton, Hestow and Well farms, to the north-east of Newton Abbot, where the walls and buildings are mainly constructed with the material, and where large boulders lie about as unmanageable obstructions. In a grass field north of Hestow large blocks, partly covered by vegetation, are scattered as though brought there by human agency for some unaccomplished purpose. Across the valley of a small stream from Hestow is the farmstead called Well, where the same rock material is met with in the walls, in obstructive boulder masses, and protruding through the ground surface. Similar rock is seen in situ, and the most suggestive position, in a field on the east of Ramshorn Down, north-west of Newton. There the masses lie together, crowning an eminence which forms a feature in the landscape.
General Description. The blocks are generally of an irregular spheroidal form, the material is of intense hardness, though somewhat brittle. The rounded shape they assume, and their generally large size, have served to preserve them from being broken up or otherwise used. The larger blocks, which have defied both the farmer and builder, range from three to six and even nine feet in diameter, and no hammer can do more with them than splinter off the angular projections, and thus render them still more unmanageable.
The dominant colour is red, which varies from a reddish yellow to the rich deep red of jasper. The stone is of very fine texture generally, and much veined by pure quartz; the contrast in colour giving it a striking appearance. Frequently cavities are met with in the mass, which are always lined with quartz crystals; the facets which terminate these inward-pointing crystals reflect the light and often sparkle like a nest of polished gems. Sometimes the fracture faces of the rock show defined patches of varying shades, suggesting a breccia composition of broken pieces of the same material with slight differences of hue, consolidated again by a cement similar in composition to the original material. A closer examination discovers, occasionally in fissures, instances of another variety of the same strong matter arranged in fine parallel layers of slightly differing colour, suggesting on a small scale the characters of the onyx and its relation thereto.
Geological Relationship. The rock masses so far described occur amidst those widely extending shales, grits, and cherts that have been given the name of the Culm series, and which form in Devonshire the representatives of that great geological age termed the Carboniferous. At Hestow, Well, and Whiteway Barton junctions occur between the Culm and other rocks. At Well and Whiteway Barton, they are inliers of the older Devonian series, while at Hestow the red sandy beds of the Permian are met with, where these altered rocks in question are exposed and protrude through the thinned-out edge of the overlying newer rock formation. Near Ramshorn Down the rock masses under review occur in the midst of the Culm beds, which around are but little altered and so more clearly indicate that these peculiar blocks belong to the Culm series.
Microscopic Characters. When examined in thin sections by aid of the microscope the rock is found to be constituted almost entirely of silica or quartz, which, however, is presented in a variety of forms. The ground mass or main component of the rock is composed of grains of detrital quartz of slightly varying sizes, but for the most part very small and stained with iron oxide. In some sections a number of minute circular spaces are noticeable. These are occupied by radiating crystals of silica in a similar manner to those met with in the unaltered Radiolarian chert beds of the Culm, and thus declaring the source of some of the material. Through the main substance of the rock veins of clear quartz of varying widths are seen traversing it in all directions, the crystals of which are noticeably fresh, lie in arranged order, and are often larger than those of the ground mass. These veins were evidently formed within the rock subsequently to its deposition and original consolidation. Nests of secondary quartz are also seen, with other irregularly shaped concretions of the same clear mineral. More rarely another form of silica is met with, in which the initial or border stage of crystal development is only just reached. This crypto-crystalline or chalcedonic condition is usually presented in radiate or fan-like forms of needle-shaped crystals, probably mixed with noncrystalline or colloid silica, out of which the chalcedonic form has segregated. Slight differences in the purity of the colloid are sufficient to mark by staining the parallelism of layers in the immature crystallization, and so give the peculiarity of marking which distinguishes the onyx in silica minerals. The purely uncrystallized colloid material can also be detected in the section by its isotropic character.
The Problem and Solution. It will be noticed that the composition of the rock is very simple, and mineralogically of very limited interest. Silica in some variety of condition constitutes the entire mass, which is given varying shades of colour by iron staining. But if the blocks in themselves and composition are of subordinate interest, their distribution and origin have long exercised the curiosity of local geologists without anything so far having been recorded to account for their erratic mode of occurrence, or respecting their other peculiarities. The problem has occupied attention for some time, and been considered from several points of view, but only one hypothesis seems to comprise all the facts and satisfy their conditions.
The key to the problem is presented in the heaped masses forming a rugged crest to a projecting shoulder of Ramshorn Down. This is surrounded by Culm material, some of which is altered, but most is in the general normal condition. The inference is that the masses in question are also composed of the same shale, grit, and chert rock matter, but very much altered by agencies that acted quite locally. The metamorphozing agent must have been that of water charged with silica in solution. Prior to the operation of the influences which brought about the peculiar local changes we are considering, the Culm series of rock deposits had been subjected to extraordinary disturbances by both lateral and vertical forces, the former of which taking place first had folded and faulted the beds to a remarkable degree. Along faults and fissures thus made the underground waters would circulate more freely, ultimately finding their way to the surface of the land, even though the same land might be itself covered by water. The next great episode in the geological history of the region was the slow but enormous upheaval of central Devon by an intrusion of fluid rock material that ultimately formed the granite mass of Dartmoor. With the welling up of this molten granite into the Culm deposits much internal heat would be brought nearer the surface. The rock that came in contact with the magma, and even within a wide border of proximity, was more or less altered by the heat of the enormous mass; and the subterranean waters for even a further distance would be affected in respect to temperature and solvent properties. Now heated water is a more ready solvent and greater absorbent of mineral matter than cold, which helps to account for the unusual amount of secondary quartz found in the rocks under notice.
Inferentially we are led to conclude that these blocks, where found, are indications of the localities through which the heated water passed or issued. Much of the material presents the character of a breccia composed of fragmentary portions of the shale, grit, and chert which form the main materials of the Culm strata. These previously crumpled and shattered rocks would offer little resistance to water under any pressure, and fragments would collect wherever fissures or spaces were formed in the water’s course. These collections of debris would be ultimately cemented together into masses by the silica-charged water, whenever the flow was sufficiently slow, and the other conditions prevailed to allow precipitation and crystallization taking place. Some portions of these rocks suggest a condition that might have arisen towards the end of the period during which they were forming. So remarkably and excessively veined are they that it would appear they must have been for a long time soaking or stewing in the solvent water, so that parts of the original rock were gradually dissolved, to be replaced by crystals of pure silica in veins and threads of extraordinary intricacy. In places the dissolved material was carried away by the moving water, thus forming fissures and hollows which, however, are always lined with rock crystals formed as the charged water became slower in movement and more saturated with the solvent material.
An occasional occurrence of the chalcedonic form of silica in these masses points to conditions of supernormal temperature and pressure in the water solvent. Water above 200° C. more readily dissolves silica and also combines with it in certain proportions forming a colloid which may be a liquid at the higher temperature, but is solid at ordinary temperatures (Van Hise). The opal is such a form of silica, while agate, onyx, and chalcedony are intermediate conditions between the purely colloid and the completely crystallized quartz. The crypto-crystalline and colloid forms of silica met with in these rock masses support the hypothesis that heated water under some pressure played an important part in endowing them with their peculiarities. Van Hise states that silicification is an earth-crust process which takes place at some depth, and but little near the surface under ordinary conditions, so that we must suppose favourable conditions of temperature and pressure prevailed during most part of the time these masses were attaining their peculiar character.
To restate our conclusions in general terms: Subsequent to the earth-crust movements which folded and faulted the Culm strata, and probably in connection with the up-welling of the granite magma therein, much heated water was circulating through portions of the upturned and fractured material that bordered the newly disturbed and rising area. Heat and pressure augmented the solvent property of the water, which became charged with silica, that it also combined with chemically to a greater or less degree, thus giving the water a cementing property where conditions of deposition and solidification prevailed in its course. The fractured rock and debris along courses of such subterranean flow would in time by silicification be consolidated into masses which, by reason of their constitution and the character of the cementing material, would be immune to all the ordinary forces of rock decay. Probably the jasperizing processes took place at some distance below the surface, and possibly beneath a depth of overspreading sea. But during the enormous lapse of time since the cessation of the rock-changing operations described, newer rock-formations have been deposited over the localities indicated, and these have again been removed by the ever-acting denuding forces of nature. The softer rock matter overlying and surrounding the silicified areas has been carried away, leaving exposed, as we see them now, the resistant masses, standing bare, defying time and the elements of decay as hardly any other rock material can do.
As similar or identical conditions would prevail in other localities bordering the granite area during the period indicated, probably rock material of similar constitution and having a like history can be found in other places round Dartmoor. Of instances brought to notice, that of Bramble Brook, Trusham, is noteworthy as being the locality from which that fine block of jasper was obtained that now stands in the entrance hall of the Albert Memorial Museum, Exeter.
The pale striped, so-called “ribbon jasper,” of Ivybridge may be mentioned as another instance of rock altered under like conditions and by the same process; while that peculiar silicification and deposition of chalcedony, called Beekite, found in and on some of the conglomeratic material of the Permian of Livermead cliffs, near Torquay, may be attributed to the modified operation of similar agencies.