Page:EB1911 - Volume 27.djvu/648

Rh Sierra Nevada on the east and the coast ranges on the west; each stream forms an alluvial fan of very gentle slope; the fans all become laterally confluent, and incline very gently forward to meet in a nearly level axial belt, where the trunk rivers-the Sacramento from the north and the San Joaquin from the south-east-wander in braided courses; their tendency to aggravation having been increased in the last half century by the gravels from gold washing; their waters entering San Francisco Bay. Kings river, rising in the high southern Siena near Mt Whitney, has built its fan rather actively, and obstructed the discharge from the part of the valley next farther south, which has thus come to be overflowed by the shallow waters of Tulare Lake, of flat, reedy, uncertain borders. A little north of the centre of the valley rise the Marysville Buttes, the remains of a maturely dissected volcano (2128 ft.). Elsewhere the floor of the valley is a featureless, treeless plain.

 

All the great systems of rock formations are represented in the United States, though close correlation with the systems of Europe is not always possible. The general geological column for the country is shown in the following table:—

Archeozoic (Archean) Group.—The oldest group of rocks, called the Archean, was formerly looked upon, at least in a tentative way, as the original crust of the earth or its downward extension, much altered by the processes of metamorphism. This view of its origin is now known not to be applicable to the Archean as a whole, since this system contains some metamorphosed sedimentary rocks. In other words, if there was such a thing as an original crust, which may be looked upon as an open question, the Archean, as now defined, does not appear to represent it. The meta-sedimentary rocks of the Archean include metamorphosed limestone, and schists which carry carbonaceous matter in the form of graphite. The marble and graphite, as well as some other indirect evidence of life less susceptible of brief statement, have been thought by many geologists sufficient to warrant the inference that life existed before the close of the era when the Archean rocks were formed. Hence the era of their formation is called the Archeozoic era.

Most of the Archean rocks fall into one or the other of two great series, a schistose series and a granitoid series, the latter being in large part intrusive in the former. The rocks of the granitoid series appear as great masses in the schist series, and in some places form great protruding bosses. They were formerly regarded as older than the schists and were designated on this account “primitive,” “fundamental,” &c. They have also been called Laurentian, a name which is still sometimes applied to them.

Nearly all known sorts of schist are represented in the schistose part of the system. Most of them are the metamorphic products of

igneous rocks, among which extrusive rocks, many of them pyroelastic, predominate. Metamorphosed sedimentary rocks are widely distributed in the schistose series, but they are distinctly subordinate to the meta-igneous rocks, and they are so highly metamorphic that stratigraphic methods are not usually applicable to them. In some areas, indeed, it is difficult to say whether the schists are meta-sedimentary or meta-igneous. The likeness of the Archean of one part of the country to that of another is one of its striking features.

The Archean appears at the surface in many parts of the United States, and in still larger areas north of the national boundary. It appears in the cores of some of the western mountains, in some of the deep canyons of the west, as in the Grand Canyon of the Colorado in northern Arizona, and over considerable areas in northern Wisconsin and Minnesota, in New England and the piedmont plateau east of the Appalachian Mountains, and in a few other situations. Wherever it comes to the surface it comes up from beneath younger rocks which are, as a rule, less metamorphic. By means of deep borings it is known at many points where it does not appear at the surface, and is believed to be universal beneath younger systems.

Locally the Archean contains iron ore, as in the Vermilion district of northern Minnesota, and at some points in Ontario. The ore is mostly in the form of haematite.

Proterozoic (Algonkian) Systems.—The Proterozoic group of rocks (called also Algonkian) includes all formations younger than the Archean and older than the Palaeozoic rocks. The term Archean was formerly proposed to include these rocks, as well as those now called Archean, but the subdivision here recognized has come to be widely approved.

The Proterozoic formations have a wide distribution. They appear at the surface adjacent to most of the outcrops of the Archean, and in some other places. In many localities the two groups have not been separated. In some places this is because the regions where they occur have not been carefully studied since the subdivision into Archeozoic and Proterozoic was made, and in others because of the inherent difficulty of separation, as where the Proterozoic rocks are highly metamorphosed. On the whole, the Proterozoic rocks are predominantly sedimentary and subordinately igneous. Locally both the sedimentary and igneous parts of the group have been highly metamorphosed; but as a rule the alteration of the sedimentary portions has not gone so far that stratigraphic methods are inapplicable to them, though in some places detailed study is necessary to make out their structure.

The Proterozoic formations are unconformable on the Archean in most places where their relations are known. The unconformity between these groups is therefore widespread, probably more so than any later unconformity. Not only is it extensive in area, but the stratigraphic break is very great, as shown by (1) the excess of metamorphism of the lower group as compared with the upper, and (2) the amount of erosion suffered by the older group before the deposition of the younger. The first of these differences between the two systems is significant of the dynamic changes suffered by the Archean before the beginning of that part of the Proterozoic era represented by known formations. The extent of the unconformity is usually significant of the geographic changes of the interval unrecorded by known Proterozoic rocks.

The Proterozoic formations have been studied in detail in few great areas. One of these is about Lake Superior, where the formations have attracted attention on account of the abundant iron ore which they contain. Four major subdivisions or systems of the group have been recognized in this region, as shown in the preceding table. These systems are separated one from another by unconformities in most places, and the lower systems, as a rule, have suffered a greater degree of metamorphism than the upper ones, though this is not to be looked upon as a hard and fast rule. The commoner sorts of rock in the several Huronian systems are quartzite and slate (ranging from shale to schist); but limestone is not wanting, and igneous rocks, both intrusive and extrusive, some metamorphic and some not, abound. Iron ore occurs in the sedimentary part of the Huronian, especially in Minnesota, Michigan, Wisconsin and parts of Canada. The ore is chiefly haematite, and has been developed from antecedent ferruginous sedimentary deposits, through concentration and purification by ground water.

The lower part of the Keweenawan system consists of a great succession of lava flows, of prodigious thickness. This portion of the system is overlain by thick beds of sedimentary rock, mostly conglomerate and sandstone, derived from the igneous rocks beneath. A few geologists regard the sedimentary rocks here classed as Keweenawan as Palaeozoic; but they have yielded no fossils, and are unconformable beneath the Upper Cambrian, which is the oldest sedimentary formation of the region which bears fossils. The aggregate thickness of the Proterozoic systems in the Lake Superior region is several miles, as usually computed, but there are obvious difficulties in determining the thickness of such great systems, especially when they are much metamorphosed. The copper of the Lake Superior region is in the Keweenawan system, chiefly in its sedimentary and amygdaloidal parts.

The Proterozoic formations in other parts of the continent cannot be correlated in detail with those of the Lake Superior region. The number of systems is not everywhere the same, nor are they everywhere alike, and their definite correlation with one another is not