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Metamorphic Limestone. Origin of the metamorphic Strata. Their Stratification. Fossiliferous Strata near intrusive Masses of Granite converted into Rocks identical with different Members of the metamorphic Series. Arguments hence derived as to the Nature of Plutonic Action. Hydrothermal Action, or the Influence of Steam and Gases in producing Metamorphism.
Taking into consideration the part played by heated waters, which will produce the most far-reaching metamorphism, dikes which cut across the bedding planes or intrusive sheets which are thrust between the strata?
Such facts may induce us to consider whether many granites and other rocks of that class may not sometimes represent merely the extreme of a similar slow metamorphism.
But the descriptions which we now possess of the Silurian deposits over immense territories in Russia and in North America, do not support the view, that the older a formation is, the more it has suffered the extremity of denudation and metamorphism. The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.
Nor will it satisfactorily explain the auriferous antimonial silica veins of the New England district, New South Wales, in which quantities of angular and unaltered fragments of slate from the enclosing rocks are found imbedded in the quartz. With respect to the metamorphism of the enclosing rocks to a greater degree of hardness, which Mr.
Graphite, a soft black mineral composed of carbon and used in the manufacture of lead pencils and as a lubricant, occurs widely in the metamorphic pre- Cambrian rocks. It is known to be produced in some cases by the metamorphism of coal, which itself is formed of decomposed vegetal tissues. Seams of graphite may therefore represent accumulations of vegetal matter such as seaweed.
At Porton, east of Truxillo, the same metamorphism which has changed the ridge of sandstone to a hard quartzite has also changed the ordinary bituminous coal into an anthracite, which is here vertical in position. The coals of Peru usually rise to more than 10,000 feet above the sea, and they are practically inaccessible.
But the descriptions which we possess of the Silurian deposits over immense territories in Russia and in North America, do not support the view that the older a formation is the more invariably it has suffered extreme denudation and metamorphism. The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained.
The subject is presented in the following order: Life as an agent in protecting, destroying, and making rocks. Cohesive Attraction. The Atmosphere as a mechanical agent. Water as a mechanical agent. Heat as an agent in volcanic phenomena, igneous eruptions, metamorphism, veins, etc. Movements of the earth's crust, plication of strata, origin of mountains, earthquakes, etc. Chemistry of Rocks.
After thirty years of study and progress, other savants, without differing from him, progressed further in the intimate knowledge of rocks; but the historian of science will not forget that Delesse was the precursor of this order of research. His studies of metamorphism will long do him honor.
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