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They are regularly interstratified with the sandstone, shale, and iron-stone of the lower coal-measures, and, on the East Lomond, with Mountain Limestone. I examined these trap-rocks in 1838, in the cliffs south of St. Andrews, where they consist in great part of stratified tuffs, which are curved, vertical, and contorted, like the associated coal-measures.
Between it and the contorted schists we passed over some soft, decomposing trap-rocks, which, both here and elsewhere, appeared to intervene between these two formations. Over the whole country between Ocotal and Totagalpa were spread many large boulders, great blocks of conglomerate, and of a hard blue trap-rock that I did not see in situ, lying on the upturned edges of the schistose rocks.
Professor Henslow express a similar opinion, regarding the origin in trap-rocks of many chalcedonies and agates. The hill, marked in Map 2 "Crater of an old volcano," has no claims to this appellation, which I could discover, except in being surmounted by a circular, very shallow, saucer-like summit, nearly half a mile in diameter.
I could not see the line of junction of these two limestones, or that of the calcareous formation and the diabasis. The transverse valley which descends from Piedras Negras and the village of San Juan, towards Parapara and the Llanos, is filled with trap-rocks, displaying close affinity with the formation of green slates, which they cover.
We may sometimes discover the relative age of two trap-rocks, or of an aqueous deposit and the trap on which it rests, by finding fragments of one included in the other in cases such as those before alluded to, where the evidence of superposition alone would be insufficient.
The trap-rocks alluded to consist chiefly of feldspathic porphyry and amygdaloid, the kernels of the latter being sometimes calcareous, often chalcedonic, and forming beautiful agates. We meet also with claystone, greenstone, compact feldspar, and tuff.
Georgian Bay, 170 miles long and 70 broad, forms the northeast portion, and lies within British jurisdiction. Saginaw, a deep and wide-mouthed bay, is the principal indentation on the western coast. The rim of this lake is composed mostly of detrital rocks, which are rarely exposed. In the northern portion of the lake, the trap-rocks on the Canada side intersect the coast.
It is in the north of Devon that these formations may best be studied, where they have been divided into an Upper, Middle, and Lower Group, and where, although much contorted and folded, they have for the most part escaped being altered by intrusive trap-rocks and by granite, which in Dartmoor and the more southern parts of the same county have often reduced them to a crystalline or metamorphic state.
Archibald Geikie in Ayrshire have shown that some of the volcanic rocks in that county are of Permian age, and it appears highly probable that the uppermost portion of Arthur's Seat in the suburbs of Edinburgh marks the site of an eruption of the same era. Two classes of contemporaneous trap-rocks occur in the coal-field of the Forth, in Scotland.
J.D. Dana, in an excellent paper on Trap-rocks "Edinburgh New Philosophical Journal" volume 41 page 198, has argued with great force, that all amygdaloidal minerals have been deposited by aqueous infiltration. I may take this opportunity of alluding to a curious case, described in my work on "Volcanic Islands," of an amygdaloid with many of its cells only half filled up with a mesotypic mineral.
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