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Updated: June 5, 2025
Hence, though both sand and mud originated simultaneously, the one near the land, the other far from it, the sands in every locality where a shore became submerged might constitute the underlying deposit. Ostrea columba. Syn. Gryphaea columba. Ostrea carinata. Chalk marl and chloritic sand. Terebrirostra lyra, Sowerby. Pecten 5-costatus. White chalk and chloritic sand.
They resemble the Folkestone beds of the latter series in mineral character, and 59 out of 156 of their fossil mollusca are common to them; but they have also 16 species common to the Gault, and 20 to the overlying Chloritic series; and what is very important, out of seven Ammonites six are found also in the Gault and Chloritic series, only one being peculiar to the Blackdown beds.
At Inchemo Island, a similar rock gradually becomes granulo-crystalline and acquires scales of mica; and this variety at S. Estevan becomes highly laminated, and though still exhibiting some rounded grains of quartz, passes into the black, glossy, slightly anthracitic schist, which, as we have seen, repeatedly alternates with and passes into the micaceous and chloritic schists.
The higher portion of the Chloritic series in some districts has been called chloritic marl, from its consisting of a chalky marl with chloritic grains. In parts of Surrey, where calcareous matter is largely intermixed with sand, it forms a stone called malm-rock or firestone. In the cliffs of the southern coast of the Isle of Wight it contains bands of calcareous limestone with nodules of chert.
Near Monte Video, where the stratification, as it would be called, of the metamorphic series is, in most parts, particularly well developed, being as usual, parallel to the foliation, we have seen that a mass of chloritic schist, netted with quartz-veins, is entangled in gneiss, in such a manner as to show that it had certainly originated in some process of segregation: again, in another spot, the gneiss tended to pass into hornblendic schist by alternating with layers of quartz; but these layers of quartz almost certainly had never been separately deposited, for they were absolutely continuous with the numerous intersecting veins of quartz.
Very large veins, with an irregular direction, often assume the aspect of short layers; and the balls of diorite heaped together in hillocks may, like many cones of basalt, issue from the crevices. Mica-slate, chloritic slate and the rocks of slaty amphibole contain magnetic sand in the tropical regions of Venezuela, as in the most northern regions of Europe.
The lowest member of the Upper Cretaceous group, usually about 100 feet thick in the S.E. of England, is provincially termed Gault. It consists of a dark blue marl, sometimes intermixed with green sand. Twenty-one species of British Ammonites are recorded as found in the Gault, of which only eight are peculiar to it, ten being common to the overlying Chloritic series.
Next below the Upper Cambrian occur strata called the Huronian by Sir W. Logan, which are of vast thickness, consisting chiefly of quartzite, with great masses of greenish chloritic slate, which sometimes include pebbles of crystalline rocks derived from the Laurentian formation, next to be described.
In this case and at Rat Island, the passage of the gneiss into imperfect hornblendic or into chloritic slate, seemed to be connected with the segregation of the veins of quartz. Greenough page 78 "Critical Examination" etc., observes that quartz in mica-slate sometimes appears in beds and sometimes in veins.
Some of the contorted strata are of a coarse mechanical structure, alternating with fine-grained crystalline chloritic slates, in which case the same slaty cleavage extends through the coarser and finer beds, though it is brought out in greater perfection in proportion as the materials of the rock are fine and homogeneous.
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