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Seeing that the interstratified porphyries are never vesicular and often not even amygdaloidal, we must conclude that the pile was formed in deep water; how then came so many fragments to be well rounded and so many to remain angular, sometimes the two kinds being equally mingled, sometimes one and sometimes the other preponderating?
Bent and undulating gypseous marl. g. Gypsum. m. Between San Caterina and Castrogiovanni, in Sicily, bent and undulating gypseous marls occur, with here and there thin beds of solid gypsum interstratified.
This sloping plain or talus blends into a perfectly flat space a few miles in width, composed of reddish impure clay, with small calcareous concretions as in the Pampean deposit, of fine white sand with small pebbles in layers, and of the above-mentioned white aluminous earth, all interstratified together.
At Worcester, in the State of Massachusetts, 45 miles due west of Boston, a bed of plumbago and impure anthracite occurs, interstratified with mica-schist. It is about two feet in thickness, and has been made use of both as fuel, and in the manufacture of lead pencils.
The saline or gypseous interstratified beds may have been produced by submarine gaseous emanations, or hot mineral springs, which often continue to flow in the same spots for ages. Beds of rock-salt are, however, more generally attributed to the evaporation of lakes or lagoons communicating at intervals with the ocean.
This deposit to the naked eye resembles the crust which often appears on weathered surfaces of feldspathic rocks; it likewise resembles those beds of earthy feldspathic matter, sometimes interstratified with porphyritic rocks, as is the case in this very district with the underlying purple claystone porphyry.
In Ayrshire, and in the western parts of Devonshire, beds of felspathic porphyry, felstone and ash are interstratified with strata believed to be of Permian age. In Devonshire these have only recently been recognised by Dr. Irving and the author as of Permian age, the strata consisting of beds of breccia, lying at the base of the New Red Sandstone.
It consists, almost purely, of carbonate of lime; the stratification is often obscure, except where rendered distinct by interstratified layers of flint, a few inches thick, occasionally in continuous beds, but oftener in nodules, and recurring at intervals generally from two to four feet distant from each other.
The bedded traps consist of feldspar porphyry, and other varieties; and the interposed Llandeilo flags are of sandstone and shale, with trilobites and graptolites. The Snowdonian hills in Carnarvonshire consist in great part of volcanic tuffs, the oldest of which are interstratified with the Bala and Llandeilo beds.
It has a base of dark green with angular crystals of felspar of a lighter green imbedded in it. It is a truly eruptive rock, occurring in intrusive bosses, or in beds interstratified with gneiss and mica-schist, and owes its various shades of green to the presence of copper.
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