These are the remarks we have been able, from observation, to make respecting the geology of a part of the United States' territory, west of the Mississippi. Relating to that part of the interior of our country which lies north-west of Lake Superior, and north of the sources of the Missouri, we have little satisfactory information. From the accurate and intelligent Mackenzie,[91] we are however able to collect a few important particulars. This enterprising voyager, it is well known, travelled from Montreal, L. C., in latitude 45° 30´, longitude 74°, in a north-west direction, to the mouth of Mackenzie's river, latitude 69°, longitude 135°; and again, at a later period, leaving his former route at the Lake of the Hills, about midway between Lake Superior and the mouth of Mackenzie's river, he ascended, in a south-west direction, the Unjegah, or River of Peace, to the Rocky Mountains, and crossing them, fell upon the sources of the northern branch of the Columbia, and from thence arrived at the Pacific, at a point a little north of the inlet of Queen Charlotte's sound. From him we learn that the Rocky Mountains continue in an uninterrupted chain, from the sources of the Missouri in the south, to a point beyond the sixty-fifth parallel of north latitude, near the mouth of Mackenzie's river. The River of Peace which he ascended in his journey to the western ocean, has its source in these mountains in about 55° north, nearly opposite to those of the great northern branch of the Columbia. Farther towards the south are the sources of the Saskatchawin, a large river, discharging itself from the {315} north-west into Lake Winnipic. The mountains in this part seem to be less elevated than those more to the south, but in other respects entirely similar. Their northern termination, according to this traveller, is in about north latitude 65°, 130° west longitude. Santa Fé in New Mexico is in latitude 36°, longitude 104° 53´ west.[92] From this it will be perceived, that the general direction of this great mountain range is nearly from north-north-east to south-south-west. We have no evidence to confirm the conjecture, which, nevertheless, is highly probable, that the principal ridges of this range consist through their whole extent of granite or other primitive rocks. Considering the stupendous character, the great elevation and uniformity of the appearance of that portion of these primitive mountains with which we are acquainted, we should be led to look for similarity of character, and similar uniformity throughout. It is commonly believed, as asserted by Maclure, that "a large mass of primitive occupies all the northern part of this continent;" and he considers the great Atlantic range of primitive, the mountains of New England, New York, and the Alleghanies, as a spur for this formation. We are not acquainted with the grounds on which this opinion is founded, but we see no reason to consider it an improbable one. Of the northern boundary of that vast formation of secondary which certainly occupies a very large portion of the interior of this continent, we are ignorant. On the south-east, its limit is the irregular border of the transition of the Alleghanies, commencing between the Alabama and Tombigbee rivers, and running north-west to Fort Anne, near Lake Champlain. From this point, a narrow and perhaps interrupted strip of secondary extends through the valley of Lake Champlain to the upper parts of St. John's river. The island and mountain of Montreal are of secondary. The country {316} also between St. John's and La Prairie is most probably secondary, as is much of that along the St. Lawrence below Montreal. From the termination of the transition near the confluence of the Alabama and Tombigbee, the secondary rocks continue on the south-west, sometimes concealed by the recent alluvial to the Black-lake river, near Natchitoches. Beyond this, the information we have is not satisfactory. From this point, turning north-west, we may for the present consider the Red river of Louisiana as the boundary of the secondary, or rather the limit of our acquaintance with this formation.
Beyond the Ozark mountains, the district between the Red river and the Canadian is occupied by the red sandstone of the salt formation, mentioned when speaking of that region, and is undoubtedly to be considered secondary. How far it extends to the west beyond the sources of Red river and the Canadian, we are unable to determine. At the commencement of the most eastern ridge of the Rocky Mountains, a few south of the high peak, and at no very great distance north from Santa Fé, the boundary again becomes determinate. From this point it runs nearly north one hundred and fifty miles, where it crosses the river Platte. From the narrative of Lewis and Clarke, we are enabled to determine with sufficient accuracy, that it crosses the Missouri not far from the Falls, in longitude 110° west. Beyond this, the little information we have, we owe to Sir Alexander Mackenzie. He informs us, that great quantities of pit coal are found about the sources of the Saskatchawin which lie near the Rocky Mountains, and between 50° and 55° north latitude. The sources of Saskatchawin are placed by this traveller near the base of the Rocky Mountains; and the coal formation which he mentions, lies on the margin of a plain extending far to the north and east. The Saskatchawin running to the east, traverses 15° of longitude, and discharges its waters into Lake Winnipic in {317} latitude 53° north. Lake Winnipic is connected by the Severn and Port Nelson rivers to Hudson's-bay. There is a water communication, interrupted by one portage, from the Saskatchawin, north-west, to the Mississippi or Churchill's river; and from thence, by the Lake of the Hills, Slave Lake, and Mackenzie's river, to the North Sea. Near the Lake of the Hills, in latitude 59°, Mackenzie found several brine springs. This, though not decisive evidence, perhaps justifies the conclusion, that secondary rocks exist in that neighbourhood. A view of the character and direction of the several large rivers which traverse the region about Hudson's Bay, of their numerous inosculations, and the number and position of the small lakes which abound in every part of it, afford, at least, presumptive evidence, that it is an extensive plain little inclined in any direction.
We may, perhaps, venture to conclude, that the secondary formation extends uninterrupted along the base of the Rocky Mountains, as far as to the Saskatchawin, where coal was observed by Mackenzie. What lies beyond is as yet unknown. From this coal formation, our boundary must for the present run in a direction a little south of east to Lake Superior, whence it may, with a few inconsiderable interruptions, follow the territorial boundary of the United States, until it arrives at 45° parallel of latitude, thence by the St. Lawrence to Montreal. The slight acquaintance we have with the country north of this line, is perhaps insufficient to justify the conjecture, that secondary formations occupy an extensive portion of that country. It is improbable, that formations of secondary extend along the base of the Rocky Mountains through their whole course, and from thence spread themselves to the east, knowing no limits but Atlantic mountains, the shores of the Gulf of St. Lawrence, and the northern ocean. We know that rocks of this formation exist about the {318} Gulf of St. Lawrence, whence coal, plaster, and sandstone, are brought to our markets.[93]
This boundary of the great formation of secondary rocks, which occupies so large a portion of the interior of our continent, includes a vast area of surface, extending through 25° of latitude, and 60° of longitude. I intend to consider that portion of it only of which the state of facts at present known enables us to speak with some degree of confidence. This portion may be conceived as occupying the area of a large triangle, the base of which is a line running from Montreal in Lower Canada, south-west to a point, near the outlet of the river Sabine, the western boundary of the state of Louisiana. The summit would be at the sources of the Saskatchawin, which are west of north from the mouth of the Sabine, and north of west from Montreal. The Rocky Mountains on the west, and the Alleghanies on the south-east mark the limits of the secondary in those directions. Its extent towards the north and north-east is as yet unknown.
In the wide space included within the lines above mentioned, we know of but one exception to the remark, that all the rocks found in place are secondary. This is the instance of the Ozark hills traversing the horizontal strata from south-west to north-east, somewhat in the manner of a whindyke. The most striking peculiarity of this range, is the prevalence among the secondary strata of crystalline substances, and what are called rocks of chymical deposition, and the alternation of these with beds and strata whose integrant particles bear evident marks of having been worn and rounded by mechanical attrition. {319} Appearances of this kind are observed in all formations of secondary rocks, but it is believed, are, in few instances, as extensive or as numerous as in this. It is well known, that the ores of lead, so abundant in many parts of this range, occur in the uppermost strata of horizontal sandstone, or in primary soils superimposed upon those sandstones. It has been suggested, that these ores of lead may have been brought down in the alluvion of rivers from some more ancient and elevated region, but any one who shall examine them in connection with the substances with which they are now found associated, will, we think, be convinced of their having been of contemporaneous origin with the sandstone. That the sparry limestones, the crystalline sandstones, and perhaps the ores of lead, (almost invariably found in the form of crystals,) have been deposited from solution in water, is highly probable; and that these depositions must have taken place in connection with circumstances not unfavourable to animal life is evident, as all these crystalline rocks abound in organised remains.
In attempting an explanation of these appearances, can any assistance be derived from recourse to the ingenious suggestion of Bakewell, that the matter of these crystalline beds and strata has been ejected from beneath the crust of the earth in a state of chymical solution. These submarine eruptions may have been numerous, and may have happened at different and remote periods; hence the alternation of rocks, consisting of particles mechanically aggregated together with those of chymical deposition. Hence the existence of metallic ores overlaying recent marine sandstones and compact limestones; for these ores, in a state of solution, may have been the matter thrown out in some of the latest eruptions.
This supposition may derive some confirmation from the well known fact that this region is still in a remarkable degree subject to subterranean concussions {320} and earthquakes. These concussions centring apparently in this range of mountains, and felt at times throughout all the western parts of the United States, are certainly too considerable in force and extent to be attributed to the operation of a cause so limited and superficial as the decomposition of beds of lignite lodged among the alluvion of the Mississippi. We do not insist upon the accounts that have been so often circulated, of the blowing, smoking, and burning mountains, said to exist in the country west of the hot springs of the Washita, because these accounts want confirmation.
Though this range of mountains has probably a nucleus of primitive rocks running through its whole extent: yet these appear but rarely at the surface. We have seen such only in the places already mentioned, and have been informed of others in Washington county, near the sources of the St. Francis, and about Lake Superior.[94]
From the information we have been able to collect, we are induced to believe that secondary rocks occupy the country on both sides of Red river, from its sources to its confluence with the Mississippi. If this be the case, the primitive of the Ozark mountains must be considered a small and insulated mass.
The inequalities of surface in this great secondary formation are considerable. It has often been called the "basin of the Mississippi," but with little propriety, since it might with equal accuracy be called the basin of the St. Lawrence, the Saskatchawin or Mackenzie's river. The form of that part of it which contains the Mississippi, is however similar to that designated by geologists as a basin-shaped cavity. As far as our acquaintance extends, it is bounded on all sides, except a narrow space at the outlet of the Mississippi, by a surface of greater elevation than itself. But whether this surface is not sometimes of secondary {321} formation is doubtful. It is dangerous to infer the existence at a former period of an insulated inland sea from any formation of secondary rocks, without being acquainted with its whole extent, with its elevation at different points, and its connexion with other rocks. On the south-east, secondary sandstones and depositions of coal are met with in some of the most elevated parts of the Alleghany mountains. The positive elevation of the primitive mountains of New England is, except at a few points, scarce equal to that of the secondary in the western parts of the state of New York. From the primitive rocks near Philadelphia, to the secondary of the Alleghanies, is an almost uninterrupted ascent. The clay-slate and granite of the Washita, occupy nearly the lowest part of the surface of the Mississippi valley. We are as yet destitute of barometrical or other observations, by which we might determine the actual height which the secondary rocks reach on the sides of the Rocky Mountains. Pike estimates the elevation of the plain at the foot of the mountains, at 8000 feet above the level of the ocean. This is doubtless overrated. We have already observed, that secondary rocks are found upon the sides of the Rocky Mountains, considerably above the level of the plain. It is probable, that this estimate of Pike's far exceeds the truth, yet any one who considers the great length and rapidity of the rivers which flow from that region, the severity of cold in winter, the rapidity with which evaporation is carried on in summer, the transparency and peculiar aspect of the sky, will be convinced that those tracts are highly elevated; and there is unquestionably good reason to believe, the secondary rocks along the eastern base of the Rocky Mountains have in many points an elevation at least equal to the summits of the Alleghanies.