Trap rock

“ The trap-rock is of a compact granular texture, occasionally running into the amygdaloid and toadstone varieties, and is rich in imbedded minerals, such as amethystine quartz, smoky quartz, carnelian, chalcedony, agate, &c., together with several of the ores of copper. Traces of copper ore in the trap-rock were first noticed on the easterly side of Keweena Point, and near the commencement of the trap formation. This ore, which is an impure copper black, was observed in a vein of variable thickness, but not in any part exceeding two and a half inches. ”

“ Quarries of trap-rock contain some striking examples. Not unfrequently a piece of trap may be found reduced, by the action of the weather, to a number of loosely-adherent coats, like those of an onion. ”

“ Again, let any one try to collect specimens of granite, porphyry, or other varieties of trap-rock, and he will find it often next to impossible to get one with a fresh fracture. He strikes with his hammer an apparently solid block, and he finds it break into small pieces, each surface of which (except that of the original exterior) is coated with dust which has somehow penetrated into little joints and fissures quite invisible to the eye. When my son and I were engaged in collecting specimens from the ridge of Samrat Fiddân, we had frequent instances of this kind ”

“ When a great flow of lava spreads out upon the surface, there is no internal lamination or stratification, and the resulting rock is usually classified as massive rather than stratified; but when a succession of flows occur, each individual flow forms a layer, and the series as a whole becomes stratiform.499 The successive flows are not usually coextensive. If the later flows of the closing stages of a period of vulcanism fail to reach as far as the earlier ones, a terraced or step-like aspect is given to the region, whence the name trap-rock (trappe, steps) is derived. ”

“ Strata of clay-slate inclined almost perpendicularly to the horizon projected at parts of the left bank, and over this clay-slate I found trap-rock. Beyond the Campaspe we crossed plains and much open land. At length on descending a little from a sort of table the trap was no longer to be seen, and we entered a wood where sandstone seemed to predominate, the strata dipping to the south-west. Fine grassy slopes extended through this forest, which was also so open that we could see each way for several miles. ”

“ It is no doubt founded in fact; the greater the thickness of the intruded trap rock, the more intense, probably, would be the squeeze, and the consequent contraction suffered by the adjacent beds. Beds of coal, too, might certainly be nearly or altogether annihilated by the intrusion of molten rock, but we cannot conceive sandstone or clunch being thus destroyed. ”

“ On my arrival I found that the lower part of the works (in which the bed of trap is situated) was not as yet sufficiently freed from water to admit of actual inspection; but being furnished with a plan of the colliery and with registers of the strata pierced through in sinking the three deepest pits, having also an opportunity of questioning several intelligent miners who had themselves worked in the colliery, and of examining many tons of trap rock and of the adjacent beds which had been brought to the surface, I think that there will be found no material error in the following statement. ”

“ For we know, in the case of lava flowing from a volcano, that a stream, when it has ceased to flow, and grown solid, very commonly ends in a steep slope, as at a, fig. 435. But, secondly, the step-like appearance arises more frequently [p.367] from the mode in which horizontal masses of igneous rock, such as b c, intercalated between aqueous strata, have, subsequently to their origin, been exposed, at different heights, by denudation. Such an outline, it is true, is not peculiar to trap rocks ”

“ The phenomenon of a dyke of trap is well exhibited in the quarries near Hartford in Connecticut, where this rock has been laid bare for a considerable depth, as it rises through a sandstone rock, instead of overlying it, as it is seen to do on the Hudson.
The trap-rocks, which are, generally speaking, of the character called by mineralogists greenstone, vary in this district of New-Jersey, from a compact basalt of homogeneous structure, to one of regular and distinct crystallization, not distinguishable in hand specimens from primitive syenite. ”

“ If the foregoing views of the origin of trap-dikes in widely extended granitic regions far from rocks of any other formation, be admitted as probable, we may further admit, in the case of a great body of plutonic rock, being impelled by repeated movements into the axis of a mountain-chain, that its more liquid constituent parts might drain into deep and unseen abysses; afterwards, perhaps, to be brought to the surface under the form, either of injected masses of greenstone and augitic porphyry, or of basaltic eruptions. ”

“ GEOLOGY.--Rapids were encountered at various points, at which there appeared large boulders of syenite and greenstone trap. No rock stratum appears in place, but from the size of the boulders, it seems probable that the trap formation crosses the bed of the Namakagun. There is no limestone--no slate. Small boulders of amygdaloid, quartz, granite, and sandstone mark the prevalence of the drift stratum, such as overspreads the upper Mississippi uplands. The weather was cloudy and overcast, producing coolness. I found the air but 64° at 2 o'clock, when the water stood at 69°. ”

“ Such masses are often called bosses, and seem not unfrequently to have been the craters of eruption whence great sheets of lava and showers of ashes were ejected far and wide over the neighbourhood. They serve to connect the intrusive traps, whose age is always more or less uncertain, with the bedded traps properly so called, the geological date of which can usually be sufficiently ascertained.
II. The bedded or contemporaneous trap-rocks consist of two well-marked kinds. There are, 1st, the melted rocks, such as greenstones and basalts and 2d, the tufas and volcanic ashes. ”

“ Trap-rocks are displayed in a great variety of situations. They often occur connected with limestone in valleys, sometimes constitute lofty ranges as on the north or left bank of the Hunter, and along the seashore at the Illawarra; they likewise cap the summit of isolated hills, but no particular place can be assigned to them with reference to the position of any other rocks. Trap forms a good soil on decomposition as is shown in the rich districts of the Illawarra, Cowpastures, Valley of the Hunter, Liverpool Plains, Wellington Valley, and Buree. ”

“ None of the beds are known to vary materially in thickness except the trap. The thickness of this latter in the Engine pit and in the Bye pit amounts to 24 feet, but in the pit B it is diminished to 12 feet, and in the pit D which is sunk down to the third coal the trap is wholly wanting. The miners themselves conclude from these facts, and apparently with reason, that the bed of trap is merely a great wedge from the green-rock fault which has intruded itself between the proper coal strata, but is by no means co-extensive with them. ”

“ In connection with the metamorphism superinduced by dykes, reference may again be made to the alteration which they themselves undergo where they have invaded a carbonaceous shale or coal. The igneous rock, as we have seen, loses its dark colour and obviously crystalline structure, and becomes a pale yellow or white, dull, earthy substance, or "white trap." ”

“ We proceeded by a circuitous and steep route about six miles, gradually ascending the mountain, which consisted entirely of an uniform porphyritic rock, broken every where into fragments and large blocks, and which in many places was so denuded of soil, as to render it a matter of astonishment how vegetation, and particularly that of the cane, should thrive so well. The far greater part of the whole island is made up of this porphyry, which by some systematicks would be considered as referable to the newest floëtz trap formation, and by others would be regarded only as a variety of lava. ”

“ A trap-dyke is a long wall of igneous matter, cutting more or less perpendicularly through the surrounding rocks. Sometimes these dykes attain a breadth of many yards, and may not unfrequently be traced for miles running in a nearly straight line over hill and valley, easily recognisable by a long smooth ridge, with the rock protruding here and there from below where the soil is thin. ”

“ The natural fissures or cleavages between the lamina run nearly in a perpendicular direction, giving the rock the columnar structure of trap or greenstone.
As the detachment proceeded, a few interesting insects and plants occurred to reward their labours. But these impenetrable and naked rocks, are the abodes of few living beings, either animal or vegetable. ”

“ All these summits, which are of various shapes, consist of trachyte, formerly called Trap-porphyry: a granular vesicular rock composed of different kinds of feldspar (Labradorite, Oligoklase, and Albite) , augite, hornblende, and sometimes interspersed mica, and even quartz. In cases where the evidence of the first outburst or eruption, or I might say where the ancient structure or scaffolding remain entire, the isolated conical mount is surrounded by an amphitheatre or lofty circular rampart of rocky strata superimposed upon each other. ”

“ The most convenient subdivision for our present purpose, however, is that which proceeds upon the origin and mode of occurrence of the trap-rocks. Viewed thus, they resolve themselves into two great groups, the intrusive and contemporaneous, both of which contain greenstones, basalts, &c.,—the sole distinction between those of the one class and those of the other, being the relation of age and mode of occurrence which they bear to the surrounding rocks. ”

“ A smart strike on the cap with a hammer will have the desired result, and by heaping additional fuel on the blazing leaves the fire soon reaches large proportions. If the young trapper should ever be so unfortunate as to find himself in the wild woods, chilled and hungry, minus matches, powder, caps, and sun glass, he may as a last resort try the following: Scrape some lint or cotton from some portion of the garment, or some Page 235 tinder from a dry stick, and lay it on the surface of some rough rock, white quartz rock if it can be found. ”

“ Such is the aspect presented by some of the more trappean regions of Scotland. But the origin of this kind of scenery must be ascribed to the effects of denuding currents in scooping out the softer strata into clefts and valleys, and leaving the harder trap-rocks in prominent relief, rather than to any great inequality of surface produced by the eruption of igneous matter; for we shall find that the throwing out of sheets of lava and showers of volcanic ashes was often a very quiet process after all. ”

“ Trap-rocks generally may be variously classified according to the aspect under which we view them. Mineralogically they are augitic, when the mineral augite enters largely into their composition; hornblendic, when the augite is replaced by hornblende; and felspathic, where felspar forms the most marked constituent. The first class includes all the dark homogeneous compounds called basalts; the second, the hornblendic greenstones, or diorites; and the third, the felstones, porphyries, and tufas. Geologically, they are beds when they are interstratified with the contiguous rocks ”

“ In one or more instances I have observed this to happen after more than a mile in extent, throughout all which space not the minutest irregularity had appeared to indicate any thing else than a perfectly conformable and alternating stratification. I have no doubt that, could such extensive exposure be oftener procured, all the instances of supposed alternation between the trap rocks and the stratified ones would prove similar to these. ”

“ In other places it swells up into bosses and rounded masses which are thrust up into the overlying rocks. The outside of these masses presents a scoriaceous or slag-like appearance; in the interior the cavities are filled with infiltrated minerals. For the first two or three feet above the trap the shales which rest directly on these igneous rocks have been intensely metamorphosed, and are scarcely to be distinguished from the trap itself. ”

“ The peculiar aspect of the rocks of this family renders the name of trap as expressive as any which can be applied, and in this sense, if it be limited to that well-known class of unstratified rocks which includes the above-named individuals and their numerous modifications, we shall escape the confusion which must otherwise ensue, and which unfortunately already exists too much. ”

“ It was gone; but the grass about the spot was beaten down, and the bushes broken. And on looking around, we discovered a trail leading off through the weeds. Following this for ten or a dozen rods, we came to a large, rough stone; and near it lay the trap, shattered and bent, with the springs broken, and the jaws gaping and powerless. The stone, too, looked newly scratched, as if from heavy blows. The trap had evidently been beaten upon.
“Some large animal,” said I.
“Bear, probably,” said Rod. “They will frequently smash up a small trap to get it off their feet.” ”