The tin veins will be of higher relative antiquity for the same reason that the "underlying" igneous formations or granites which are visible to man are older, on the whole, than the overlying or trappean formations.
We can also prove that subsequently both the granitic and trappean rocks underwent great aqueous denudation, which they probably suffered during their emergence from the sea.
Relation ofTrappean Rocks to the products of active Volcanos.
If there be any difference of date, it would seem that the fine-grained granite must be newer than the trappean rocks.
In the southern part of Devonshire, trappean rocks are associated with New Red Sandstone, and, according to Sir H.
C] The vast thickness of contemporaneous trappean rocks of lower Silurian date in North Wales, explored by our government surveyors, has been already alluded to.
The Caradoc sandstone, which abuts against the trappean chain called the Caradoc Hills, in Shropshire.
They never repose on the volcanic or trappean formations, nor on strata containing organic remains.
Relation of trappean Rocks to the Products of active Volcanoes.
To a certain extent, however, there was a real distinction between the trappean formations and those to which the term volcanic was almost exclusively confined.
It is worthy of remark that, when it occurs under the form of trappean tuff (volcanic ashes of De la Beche), as in the crest of Snowdon, the peculiar species which distinguish it from the Llandeilo beds are still observable.
By degrees familiarity with the products of active volcanoes convinced geologists more and more that they were identical with the trappean rocks.
If the term "overlying" can in this instance be applied to a Plutonic rock, it is only in proportion as that rock begins to acquire a trappean aspect.
In its course it affords a good exemplification of the passage from the trappean into the Plutonic, or highly crystalline texture.
These tuffs, and the associated trappean rocks, must not be compared to lava and scoriae which had cooled in the open air.
Numerous dikes likewise consist of this greenstone porphyry; others are formed of various fine-grained trappean rocks; but very few of claystone porphyry: I saw no true basaltic dikes.
Westward of this main range, the metamorphic schists are foliated, though less plainly, in the same direction, which is likewise common to the zone of old erupted trappean rocks, forming the outermost islets.
Dikes are frequent in the metamorphic schists of the Chonos Islands, and seem feebly to represent that great band of trappean and ancient volcanic rocks on the south-western coast of Tierra del Fuego.
In the valley of the Marmaraya, a few miles south of the Tapas, a band of trappean and amygdaloidal rock is interposed between a hill of granite and an extensive surrounding formation of red conglomerate, which (like that at the foot of the S.
I observed boulders of greenstone in several places; and on the islet of Villegagnon, and likewise on the coast some miles northward, two large trappean dikes.
According to some observations communicated to me by Mr. Fox, the coast from Rio de Janeiro to the mouth of the Plata seems everywhere to be granitic, with a few trappean dikes.
Some of the islets are composed entirely of the sedimentary, others of the trappean rocks, generally, however, with the sandstone, cropping out on the southern shores.
By what train of investigations were geologists induced at length to reject these views, and to assent to the igneous origin of the trappean formations?
The lodes are not ribboned, but consist of quartz, jointed across from side to side, exactly like trappean dykes.
George Town is a straggling village lying two miles within the entrance of the Tamar; in its neighbourhood were found greenstone, basalt, andtrappean rocks.
I noticed here a trappean dyke, but the general formation of this end of King Island exactly corresponded with that about Captain Smith's house, which shows that it is a continuous ridge of granite.
The latter is a singular cliffy mass of trappean rock, rising abruptly from the water till its flattened crest reaches an elevation of 490 feet.
The hydrated volcanic rocks, such as the basalts and trappean rocks in general, continue to produce effects of metamorphism, in which heat operates, although its influence is inconsiderable, water being much the more powerful agent.
Argillaceous rocks, like all others, are subject to metamorphism when they come in contact with eruptive trappean rocks.
The unstratified rocks are Granite, Hypersthene rock, Limestone and Serpentine, and theTrappean and Volcanic rocks.
We may therefore regard the trappean rocks as the lavas of submarine volcanoes.
The volcanic rocks have become vitreous, the granitic are crystalline, and the trappean are intermediate in structure, coinciding with the circumstances of pressure and rate of cooling under which they have severally been formed.
What circumstances distinguish the trappean from other volcanic rocks?
The trappean rocks may, in a general classification, be considered as volcanic.
The trappean rocks occur more or less abundantly in all countries.
The trappean rocks are composed of felspar, mingled intimately and in small particles with augite or hornblende.
Sometimes both varieties meet in the same fragment, as we observed also in the trappean porphyries of the valley of Mexico.
A similar disengagement of vapours, joined to the elastic force of the gases, which penetrate strata softened and raised up, appears sometimes to have given great extent to the caverns found in trachytes or trappean porphyries.
Are these strata, contained in the trappean mountains, owing to muddy irruptions, or must we consider them as sediments of water, which alternate with volcanic deposits?
This peculiar form, and the symmetrical arrangement of a few cones which surround the Brigantine, made me at first think that this group, which is wholly calcareous, contained rocks of basaltic or trappean formation.
As regards the geological age of this great volcanic series much uncertainty exists, owing to the absence of marine forms in the inter-trappean beds.
But outside the central trappean area volcanic foci are numerous, as in Cutch, the Rajhipla Hills and the Lower Narbudda valley.
We this day crossed several fine running streams and forests of box and bluegum growing on ridges of trappean conglomerate.
We first crossed more hills of the trappean conglomerate on which grew ironbark eucalypti and box.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "trappean" in a variety of sentences. We hope that you will now be able to make sentences using this word.
