AGE OF UNSTRATIFIED ROCKS.

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strata, may be called, (to adopt the phraseology of Mr. Lyell,) the Primary Plutonic : those during the deposition of the secondary strata, b, b, whose veins do not enter the tertiary series, the Secondary Plutonic : those during the deposition of the tertiary strata, c, c, the Tertiary Plutonic and lava from active volcanos d, d, the Recent Plutonic.

Section of the Relative Age of the Unstratified Rocks.

Descr. In reality, however, we do not find varieties of unstratified rocks whose veins are thus distinctly confined to each of the great classes of rocks, though there is evidence that volcanic agency was active during all the periods of their deposition. But the same igneous rock appears to have been ejected at different epochs. Granite, however, seems to have greatly predominated during the first or primary period; and is found only occasionally during the secondary period; though in a few instances (at Weinbohla) syenitic granite has been protruded through the chalk, but never among the tertiary strata. Por

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38

ANCIENT AND MODERN UNSTRATIFIED ROCKS.

phyry appears to have been mostly confined to the period of the latest primary, and the older secondary (transition) rocks. Trap rock predominated in the secondary and tertiary periods, while volcanic rocks, in the common acceptation of the term, began to be protruded during the tertiary period, and continue to the present time.

Rem. It is obvious, that with the exception of lava, the above rule for determining the relative age of the stratified and unstratified rocks, does not show us when the latter began to be erupted, but only when their eruptions ceased.

Inf. From the phenomena that have been detailed respecting the unstratified rocks, it has been inferred that the condition of the earth, both internal and external, must have been different at different epochs; so as at one period to be peculiarly favorable for the production of granite and syenite, at another of porphyry, at another of trachyte, at another of basalt, and finally at another of the lava of extinct and active volcanos; and hence, that the older igneous rocks, (ex. gr. granite, syenite, &c.) are no longer produced, except perhaps in the deep recesses of the earth.

Proof 1. The greater abundance of granite and syenite associated with the primary than with the newer strata, and of trap and volcanic rocks with the higher formations. 2. The almost entire identity between the chemical constitution of granite and the primary stratified deposits indicates some general and common cause for the origin of both while the difference of ultimate constitution between granite and the newer stratified rocks, particularly in the greater quantity of lime in the latter, indicates a difference of origin. 3. The gradual and insensible passage, on an extensive scale, of granite into gneiss, hornblende, slate and mica slate, indicates some general cause for their production, and that the diversity existing between them, has resulted from slightly modifying circumstances: while no such transition of any consequence between granite and the newer stratified rocks has ever been discovered. 4. Granite and the trap rocks differ so much in chemical constitution, as to show that they must have originated from different masses of matter. Thus, granite contains about 20 per cent. more of silica than greenstone ; about 3 per cent. less of alumina; 8 per cent. less of magnesia; 7 per cent. less of lime; and two per cent. less of oxide of iron. 5. The correspondence between the chemical composition of the fossiliferous stratified and the trappean and volcanic rocks; that is, we find in both classes a diminution of silica and an increase of alumina, magnesia, and lime. 6. In consequence of containing much more of silicate of lime, the trap rocks are more fusible than the granitic: so if we admit that the internal temperature of the earth has diminished, we might expect that the former would remain in a melted state after the latter had all been consolidated. De la Beche's Theoretical Geology, p. 305.

Opposite Hypothesis. "Granitic and trap rocks pass into each other, and are merely different forms which the same elements have assumed according to the different circumstances under which they have consolidated from a state of fusion."" The great pressure of a superincumbent mass, and exclusion from

IS GRANITE NOW PRODUCED?

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contact with the atmosphere and perhaps with the ocean, are some of the conditions, which may be necessary to produce the granitic texture." "They, (Plutonic rocks) may have been produced in nearly equal quantities, during equal periods of time, from the earliest to the most modern epochs, instead of diminishing in quantity at each successive epoch, as some geologists pretend." Lyell's Principles of Geology, Vol. 2. p. 481, 482, 483.

Geological Maps and Sections.

Descr. Common or physical maps form the basis of geological ones: and when the former are inaccurate, the latter must be so too. The chief difference between them is, that on a geological map the different rocks found in the region delineated are shown either by dots, crosses, circles, &c. or more usually by colors. The only exception is, that when the nature of the subjacent rock can be determined, diluvium is usually omitted.

Descr. Some geological maps designate only the classes of rocks but these are very imperfect, and the best maps show the extent of each rock.

Descr. The dip of the strata, (which of course determines the strike,) is sometimes shown upon a geological map. This is usually done by an arrow, which points in the direction of the inclination. If the strata are perpendicular, it may be represented by the lines crossing at right angles; one of which is shorter than the other. If the two lines are equal, so as to form a cross they indicate horizontal strata. An anticlinal axis is shown by a straight line crossed by an arrow with two heads. Where the strata undulate a good deal, the body of the arrow may be crooked. De la Beche's Manual of Geology, p. 602.

Rem. The best geological maps hitherto published in Europe, are Greenough's Map of England, and Wales; Elie de Beaumont's and Dufrenoy's France; Hoffman's Northwestern Germany; and Oeynhausen, La Roche, and Von Dechen's Rhine. In this country, Maclure's Geological Map of the United States, although it exhibits only the great classes of rocks, yet considering the early period at which it was executed, must be regarded as very valuable and a work of immense labor. The geological surveys now going on in most of the States, have already produced Maps of Massachusetts, Rhode Island, New Jersey, and Tennessee; and others are in a state of great forwardness.

Descr. A Geological Section represents a vertical cut in the earth's crust, so as to exhibit to the eye the rocks in their natural and relative situation. The most valuable Sections of this sort are those copied from cliffs, on the sea coast, or the banks of rivers. But usually it is necessary to construct them from what we can learn of the rocks and their dip at the surface; presuming that they continue the same to the depth of the Section. Such Sectious, therefore, are somewhat ideal: but if carefully constructed, we may be sure that we are not far from the truth.

Descr. It is usually necessary to employ two scales in constructing Sections: one for heights, and the other for horizontal distances: otherwise the Sections must be of great extent, or the heights would be scarcely perceptible. On the other hand, two scales produce distortion: so that great caution is necessary, not only in the construction of Sections, but in drawing inferences from them.

SECTION V.

PALEONTOLOGY, OR THE SCIENCE OF ORGANIC REMAINS.

Def. In all the stratified rocks above the primary, more or less of the relics or traces of animals and plants occur, sometimes called petrifactions, but more commonly, Crganic Remains. Def. That branch of Geology which gives the history of these remains, was formerly denominated Oryctology ; but is now called Paleontology.

1. General Characters of Organic Remains.

Descr. In a few instances, animals have been preserved entire in the more recent rocks.

Example. About the beginning of the present century the entire carcass of an elephant was found encased in frozen mud and sand in Siberia. It was covered with hair and fur, as some elephants now are in the Himalayah mountains. The diluvium along the shores of the Northern Ocean, abounds with bones of the same kind of animals: but the flesh is rarely preserved. Cuvier's Essay on the Theory of the Earth, p. 253, New York, 1818. De la Beche's Manual of Geology, p. 200. In 1771, the entire carcass of a rhinoceros was dug out of the frozen gravel of the same country. Bakewell's Geology, p. 331.

Decrip. Frequently the harder parts of the animal are preserved in the soil or solid rock, scarcely altered.

Rem. Many well authenticated instances are on record, in which toads, snakes, and lizards, have been found alive in the solid parts of living trees, and in solid rocks, as well as in gravel, deep beneath the surface. But in these instances the animals undoubtedly crept into such places while young, and after being grown, could not get out. Being very tenacious of life, and probably obtaining some nourishment occasionally by seizing upon insects that might crawl into their nidus, they might sometimes continue alive even many years. But such examples cannot come under the denomination of organic remains. See an interesting paper on this subject by Dr. Buckland, in the American Journal of Science, Vol. 23. p. 272.

Descr. Sometimes the harder parts of the animal are partially impregnated with mineral matter; yet the animal matter is still obvious to inspection.

Descr. More frequently, especially in the older secondary

rocks, the animal or vegetable matter appears to be almost entirely replaced by mineral matter, so as to form a genuine petrifaction.

Rem. Probably in every case, however, a chemical process would show the presence of considerable organic matter. Parkison's Organic Remains of a Former World, Vol. 2. p. 234.

Descr. Sometimes after the rock had become hardened, the animal or plant decayed and escaped through the pores of the stone, so as to leave nothing but a perfect mould.

Descr. After this mould had been formed, foreign matter has sometimes been infiltrated through the pores of the rock, so as to form a cast of the animal or plant when the rock is broken open. Or the cast might have been formed before the decay of the animal or plant.

Descr. Frequently the animal or plant, especially the latter, is so flattened down that a mere film of mineral matter alone remains to mark out its form.

Descr. All that remains of an animal sometimes is its track impressed upon the rock.

Descr The mineralizer is most frequently carbonate of lime; frequently silica, or clay, or oxide, or sulphuret of iron, and sometimes the ores of copper, lead, &c.

2. Nature and Process of Petrifaction.

Def. Petrifaction consists in the substitution, more or less complete, by chemical means, of mineral for animal or vegetable matter. De la Beche's Theoretical Geology, Chapter. 13. Descr. The process of petrifaction goes on at the present day to some extent, whenever an animal or vegetable substance is buried for a long time in a deposit containing a soluble mineral substance that may become a mineralizer.

Exam. 1. Clay containing sulphate of iron, will, in a few years, or even months, produce a very perceptible change towards petrifaction in a bone buried in it. Bakewell's Geology, p. 19. Some springs also, hold iron in solution; and vegetable matters are in the process of time thoroughly changed into oxide of iron. This is seen often where bog iron ore is yearly depositing.

Ex. 2. M. Goppert placed fern leaves carefully in clay, and exposed the clay for some time to a red heat, when the leaves were made to resemble petrified plants found in the rocks. Wonders of Geology, Vol. 2. p. 561.

Hypothetical Example. 3. M. Patrin and Brongniart suggest that the petrifying process may sometimes be effected "suddenly by the combination of gaseous fluids with the principles of Organic Structures." Wonders of Geology, Vol. 2. p. 559. Some facts render this probable. For stems of a soft and succulent nature are preserved in flint; and the young leaves of a palm tree in a state just about to shoot forth, have been found completely silicified. Lyell's Elements of Geolgy, p. 89.