Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: June 27, 2025
Among these may be mentioned many Hippurites, Sphaerulites, and other members of that great family of mollusca called Rudistes by Lamarck, to which nothing analogous has been discovered in the living creation, but which is quite characteristic of rocks of the Cretaceous era in the south of France, Spain, Sicily, Greece, and other countries bordering the Mediterranean.
It is of a very complicated nature, and consists of numerous thin, slightly tortuous layers of a pale-coloured feldspathic stone, often passing into an imperfect pitchstone, alternating with layers formed of numberless little globules of two varieties of obsidian, and of two kinds of sphaerulites, embedded in a soft or in a hard pearly base.
OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper ones are externally marked with parallel ridges. The internal radiating structure of the lower ones, is much too plainly represented. One of the commonest transitional rocks deserves in several respects a further description.
These layers sometimes interbranch and form loops; but I did not see any case of actual intersection. They form the passage from the perfectly glassy portions, to those nearly homogeneous and stony, with only an obscure concretionary structure. In the same specimen, also, sphaerulites differing slightly in colour and in structure, occur embedded close together.
The sphaerulites and the little nodules of obsidian in these rocks so closely resemble, in general form and structure, concretions in sedimentary deposits, that one is at once tempted to attribute to them an analogous origin.
Stokes' fine collection of obsidians from Mexico, I observe that the sphaerulites are generally much larger than those of Ascension; they are generally white, opaque, and are united into distinct layers: there are many singular varieties, different from any at Ascension.
The sphaerulites are either white and translucent, or dark brown and opaque; the former are quite spherical, of small size, and distinctly radiated from their centre.
Several causes appear capable of producing zones of different tension, in masses semi-liquified by heat. In a fragment of devitrified glass, I have observed layers of sphaerulites which appeared, from the manner in which they were abruptly bent, to have been produced by the simple contraction of the mass in the vessel, in which it cooled.
Radiolites foliaceus, d'Orbigny. Syn. Sphaerulites agarici-formis, Blainv. Hippurites organisans, Desmoulins. Upper chalk: chalk marl of Pyrenees? Young individual; when full grown they occur in groups adhering laterally to each other. b. Upper side of the upper valve, showing a reticulated structure in those parts, b, where the external coating is worn off. c.
The dark brown sphaerulites are less perfectly round, and vary in diameter from the twentieth to the thirtieth of an inch; when broken they exhibit towards their centres, which are whitish, an obscure radiating structure; two of them when united sometimes have only one central point of radiation; there is occasionally a trace of or a hollow crevice in their centres.
Word Of The Day
Others Looking