Imágenes de páginas
PDF
EPUB

the water, were daily observed (except from the 3d to the 8th of August), when the water was calm, and a steady progressive rise noticed by means of them, the lowest being first covered and others in succession. It has been observed on this lake, that the water is lowest in spring and highest in autumn. This is readily explained by the fact that in winter most of the ordinary supplies of water from the drainage of the surrounding country, are cut off by being converted into ice and snow, while evaporation from the surface of the lake by the dry northern winds continues to carry away a very sensible quantity of water. During the spring and early part of summer* the snow and ice melt, and the accumulated stores of winter, flow into the lake in greater quantity than to compensate for the evaporation and the drainage at the outlet. The summer of 1846 was remarkably dry and warm; that of 1847 more than usually cold and wet. A small canal was cut some years since, from the head of the rapids at the Sault St. Marie, to supply the government saw mill near the foot of the rapids with water, and boats used to navigate this canal to the saw mill. In 1845 a little water entered this canal, perhaps eight inches to twelve inches in depth. In 1846 and 1847 the water did not ordinarily come within a foot or more of the level of its bottom.

Ancient Levels of the Lake.—During a century past, the waters of Lake Superior cannot have been more than four feet above the level of the summer of 1847, for any considerable length of time. This is evident from the growth of trees of two feet in diameter on Porter's Island, within one hundred yards of the Government House that would have died had the ground around been inundated for any length of time.

The evidences of higher levels of this lake in times more remote, but during the modern epoch, are numerous and striking. They consist in the beaches of shingle, and sand, and gravel, in successive elevations. The shingle and pebble beaches may be seen, well characterized, between Copper Harbor and Lake Fanny Hooe at Fort Wilkins, from the lake beach to a height of about forty feet. The sand beaches and dunes, may be seen at the mouths of Huron and Pine Rivers, and near Presque Isle, between the Presque Isle River and Point Abbaye ; also east and west of Eagle River, from Eagle Harbor to the Portage, for about twenty-five miles; also most of the distance between Elm River, Sleeping River, Misery, Flint-steel, Fire-steel, Ontonagon and Iron Rivers, where the rocky coast is not too high to break the force of the waves when the lake was above its present level.

• On the 2014 of June, 1847, snow still remained in the swamp on Porter's Isiand, two feet in depth, over a small area where the evergreen trees were so thick that the sun's rays could scarcely penetrate.

The most remarkable dunes or hills of blown sand, observed, are those w.s.w. of Eagle Harbor, between there and Sand Bay, which are fifty feet or sixty feet high on the coast, and much higher in the interior, near the base of the hills; those near Eagle River to the west four to five miles ; and those at the mouth of Pic River on the Canada shore. Dunes of twenty feet to forty feet high are common in almost every part of the lake coast where sand beaches are formed, and where the strong dry winds can sweep over a considerable length of beach. The highest and most remarkable dunes are so situated as to demonstrate that the westerly and northwesterly winds are the prevailing ones, that blow with force enough to drift much sand. Mr. Schoolcraft has described remarkable dunes on the lake coast, three hundred feet high,* between Grand Island and White Fish Point. All who have coasted that shore, have observed them.

An estuary deposit at the mouth of Pic River, may also be adduced as an evidence of a former higher level of the lake. This is about twenty feet to forty feet above the lake, nearly level, and where the river was cutting it away, great numbers of freshwater bivalve shells were found, such as are now living in great numbers in the adjacent waters, and occur on the beach. This deposit is not blown sand, as it is nearly level, while that near the beach is piled in hills sixty feet to seventy feet high, and continually encroaching upon the level estuary deposit, burying the forest on the landward side in its progress. * Lake Superior, so far as is known, does not show any indication of having had any other outlet than that through which its surplus water now flows, and so far as the facts observed justify any conclusion, we may infer, that the lowering of level indicated by the preceding facts, may be ascribed to the gradual wearing down of its outlet at the Sault St. Marie. The rocks at this place are gray and red variegated sandstones highly indurated, which wear away with extreme slowness. They are nearly horrizontal in position, and dip slightly to the west and northwest. The outlet in former times, when the lake was twenty feet or thirty feet higher, must have been two miles in width, but shallow; the flat lands at the Portage of the Sault St. Marie, consist of gravel and boulder of almost every variety of rock found on the shores of Lake Superior, intermixed with loam, and these materials rest on the flat and often smoothed surface of the subjacent sandstone. 1. The facts considered as demonstrating a water level some hundred feet above the present level will be adduced in another article on the geology of that region.

* Am. Journal of Science, vol. xliv, p. 369.

Art. II.—Contributions to the Geology of Texas ; by Dr.

FERDINAND REMER.

At the time when I wrote the short sketch of the geology of Texas, contained in a previous number of this Journal,* I had seen only a comparatively small portion of the country. I have since extended my observations over a much larger surface and, profiting by some peculiarly favorable circumstances, I have become acquainted with sections of the country generally considered inaccessible on account of the dangerous character of the Indian tribes by which they are inhabited.

I have collected a sufficient number of facts for a geological map of the whole state, which will be true at least in all its general features. My collections of fossils will serve to test the correctness of the observation and the inferences drawn from them. They contain a considerable number of new forms, chiefly from the cretaceous formation, which require to be described.

At present I wish only to present a short account of the general results which I have derived from my geological survey of the country.

An ideal line drawn from Presidio de Rio Grande on the Rio Grande in a N.E. direction, and crossing the San Antonio River at the town of the same name, the Guadaloupe at New Braunfels, (the German settlement,) the Colorado at Austin, the Brazos at the falls of this river, the Trinity below its forks, and reaching from there to the Red River in the same n.E. direction, divides the tertiary strata and the diluvial and alluvial deposits (of the level and rolling part of the country) from the cretaceous and older formations (of the hilly and mountainous sections).

The few remarks to be made about the former region, are first that the tract of level country which extends like a broad belt along almost the whole coast of Texas, is diluvial and partly alluvial in character. Its small elevation of a few feet only above the level of the sea, and its perfectly level surface, indicate, at once, the recent origin of the soil. The fossil remains, found in many places in the deposits of clay and sand, prove their modern age still more conclusively. At the head of Galveston bay and even near the town of Houston, I found at a height of twelve to twenty feet above the general level of the bay, large deposits of shells of Gnathodon, a bivalve mollusc, which lives abundantly in the brackish waters along the coast of the Mexican Gulf, and in the bay of Galveston particularly. Some few oyster shells of the common kind occur in these deposits of half fossilized Gna

[ocr errors]

thodon shells, but there are no shells different from those now living in the bay. Every thing tends to the supposition that the conditions of climate, etc., at the period when these deposits along the coast of Texas were formed, did not differ materially from the present, except that a change in the relative level of land and sea has since taken place.

To the diluvial period must be likewise referred the deposits of clay and sand which form the banks of the Brazos and probably all the other large rivers of the country. Mr. Hough, a gentleman residing at San Felipe, has discovered in the muddy banks of the Brazos near his place of residence, many fossil bones of extinct species of mammalia, and has made a valuable collection of them, which I had an opportunity to examine when it was exhibited about two years ago at Galveston. It contains bones of mastodon, megalonyx (claw bones), tapir, and of a gigantic and undescribed species of ox.*

To the diluvial age of the globe must be further referred the deposits of gravel and sand, which form a broad belt of barren or poor land covered with pine and post-oak timber, in the - rolling" or undulating portion of Texas, and extending from west to east across a considerable part of the country. Following up the Colorado from Columbus to Bastrop, or the Guadaloupe from Gonzales to Seguin, we pass directly across this belt. The gravel is mostly composed of pebbles of silex evidently derived from decomposed cretaceous strata. Within the limits of this gravel formation, fossil wood of dicotyledonous trees in smaller or larger fragments is found almost every where. In some localities it is particularly abundant, and whole trunks are occasionally met with. I have sent to Europe the lower part of a trunk, about three and a half feet in diameter, weighing about six hundred pounds, and showing distinctly the beginning of the ramification of the roots and most beautifully the fibrous internal structure of the wood. This specimen was discovered together with many smaller ones, in the banks of a small creek near the town of Boonville on the Brazos. When I wrote my former paper, I was not sure about the formation in which this fossil wood was originally deposited. I am now perfectly convinced that it is derived from cretaceous strata, having afterwards found pieces of it among cretaceous fossils at localities where for hundreds of miles around, there are no other but cretaceous strata, and no traces of diluvium or drift are met with.

Strata, belonging decidedly to the tertiary period, I did not see at all during the first part of my stay in the country, and I was inclined almost to doubt their existence in Texas altogether, although this would have been against the general analogy of the

* This Journal, volume i, ii Ser., p. 244.

other southern states.' While on a tour to the upper Brazos, I discovered in the neighborhood of the town of Caldwell, strata of a ferruginous sandstone with numerous and well preserved tertiary shells. Crossing afterwards the Brazos not far from this town, I had a still better opportunity to see this formation along the steep banks of the river. It consists of alternating strata of brown ferruginous sandstone and of dark colored plastic clay, both teeming with fossils. Unfortunately, the circumstances did not allow me to make a complete collection of them; the few, however, which I gathered are sufficient to prove that those strata belong to one of the older divisions of the tertiary period. I have good reason to suppose that these same tertiary deposits have a wide range in the eastern part of Texas, though I am unable to give their exact limits. Tertiary fossils from Nacogdoches seem to indicate that the deposits of the Brazos extend as far as there.

Cretaceous formation.-We come next to the cretaceous strata, which of all the stratified formations take the most important part in the geological constitution of Texas and chiefly her upper hilly part. The immense tract of land which extends from the above mentioned line, connecting the Rio Grande with Red River, to the head waters of the Colorado and the other large rivers of Texas, is occupied entirely by cretaceous deposits, except a belt of silurian and carboniferous strata and a mass of granitic rocks, both covering comparatively a small area.

When we examine first the mineralogical constitution of these cretaceous rocks, a striking difference from other deposits of the cretaceous period on the North American continent at once becomes apparent; for whereas these latter, on the whole Atlantic coast, are almost entirely composed of loose and incoherent materials, the cretaceous strata of Texas constitute mostly compact and hard rocks, some of them equalling in compactness the hardest strata of more ancient secondary formations. A calcareous character is very commonly observed ; in fact, I have not seen any sandstones or strata of clay in the whole series. Generally speaking, there is an alternation of compact siliceous limestones and less compact beds of either pure, or marly, limestone. The former contain the silex as well diffused through their whole mass, as in separate concretions or nodules. The siliceous character of these rocks, excluding the decomposing action of the atmosphere, almost entirely produces the general dry and barren aspect of the country which they occupy. Every where in the mountainous region, north from Austin or San Antonio de Bexar, on both sides of the Piedernales and San Saba Rivers, it is only in the valleys that a fertile stratum of soil is found; on the heights of the table land the bare rock appears almost every where at the surface, hardly supporting the scanty growth of grass and some scattered specimens of stunted live-oak and post-oak trees.

« AnteriorContinuar »