the intrusives are regarded simply as parts of the main mass which have become perfectly fluid and therefore locally take on eruptive forms. Lawson declines to carry the term metamorphic over to the plastic material and speaks of it as a subcrustal magma. As the real character of such granite-gneiss-schist complexes is a question which concerns not only the Colorado ranges, but almost every other preCambrian region of North America, the discussion of this question is deferred for the general chapter. It is, however, plain that in the mountain ranges of Colorado is a thoroughly crystalline, intricate, fundamental complex like that found in most of the pre-Cambrian areas already considered.

Besides these crystallines there are at least two areas in which unmistakable pre-Cambrian clastics are present. These are the districts of Big and Little Thompson, South Boulder, Coal and Ralston creeks in the Front range, and the district of the Quartzite mountains in the San Juan region. It is also probable that the quartzites and mica-schists of northern New Mexico, described by Stevenson, is a third series, although the relations of these beds to the other rocks are not indicated. The great beds of white quartzite and the granite-conglomerate in the neighborhood of Santa Fe strongly suggests that here is a clastic series, the granite-conglomerate probably being a recomposed rock. A fourth great series which possibly falls among the pre-Cambrian clastics is that seen by Cross in the neighborhood of Salida.

In the Front range the descriptions of Marvine and Lakes agree that there is an apparent gradation from the clastic quartzite and mica-schist series to the gneisses and gneissoid granites, although Lakes states that the transition is somewhat abrupt. Two explanations may be applied here: First, there may be a real physical break which has not been detected between the clastics and the crystallines. In favor of this hypothesis is the fact that quartzites are found nowhere else in the vast area of the Front range, and that nowhere else are there any rocks which are even described as having any evidence of fragmental character, unless foliation be taken as such evidence. If this clastic series and the great complex of granite, gneiss and schist are of the same origin and age, it is certainly strange that nowhere except in this very restricted area do beds of quartzite or quartz-schist occur. Also the presence of genuine granitic pebbles in the conglomerates at least shows that there existed an earlier granite from which débris was derived. Second, those who believe that the fine-grained gneisses, completely crystalline schists and granites are all really of eruptive origin, the foliation being but evidence of powerful dynamic action, will probably maintain that this clastic series is the more ancient one and has been cut and metamorphosed by the intrusion of the igneous rocks.

In the Quartzite mountains the evidence that the fragmental slate and quartzite series is more retent than the granite-gneiss-schist com plex is far more weighty. Here an intricate complex of irregularly

banded gneisses, granitoid gneisses and granites are cut by foliated dikes of hornblende-schist, which never penetrate the fragmental series. At one of the places along the Animas at which the quartzite is separated by a very short interval from the granite complex there is a sharp discordance in its foliation and a series of sharply folded anticlines and synclines of quartzite, which with considerable certainty indicate the bedding of the latter group. Also, while the quartzite series inclines at a steep angle and is in places sharply folded, it upon the whole has not suffered any such profound and repeated dynamic movements as are exhibited by the granite-gneiss complex. If the latter series be taken as sedimentary its complete metamorphism argues its greater age; and if it be taken as wholly eruptive its present implicated character, with strongly developed schistose structures, denoting profound metamorphism, indicates a history much longer than the one revealed by the quartzites. This great quartzite and slate series can then with a considerable degree of certainty be regarded as much later in age than the granite-gneiss-schist complex. Also it is far more ancient than the Carboniferous, because near Ouray the Trias conformably above the Carboniferous rests in a nearly horizontal position upon the upturned, nearly vertical, truncated edges of the quartzite. On general structural and lithological grounds it may with great probability be referred to the Algonkian. Of the two other areas of clastic rocks too little is known to offer any suggestions as to their age or relations.

A part of the so-called granite (diorite?) of the Elk mountains and a part of that of the Sangre de Cristo range is plainly an eruptive of later age than the Cambrian, and therefore it does not properly fall within the province of this review.

Whether there are any pre-Cambrian rocks in the La Plata mountains is uncertain. The small area of metamorphosed rock bears such relations to the eruptives as to suggest that their present condition may be due to contact metamorphism.

SECTION VII. ARIZONA AND WESTERN NEW MEXICO.

LITERATURE.

POWELL, ,28 in 1874, states that below the Carboniferous is a succession of nonconformable shales, sandstones and limestones, the greatest thickness of the beds being a little more than 10,000 feet. The beds are traversed by dikes of trap or greenstone and irregular layers of the same eruptive material are found in places between these nonconformable rocks and the overlying beds of Carboniferous age. Provisionally these sedimentary rocks are called Devonian, and Silurian. Still underlying these is an extensive series of metamorphic crystalline schists, in some places yet showing faint traces of the original stratification, but usually these are so degraded that the total thickness of the

beds was not determined. In places they constitute about a thousand feet of the altitude of the walls. These beds are traversed by dikes of granite, and beds of granite are found which are believed to be intrusive, hence of igneous origin. In some places the evidence is complete. An extensive period of erosion separates these schists and granite from the overlying Silurian and Devonian rocks.

In the Grand canyon are the records of an extensive period of deposition in the schists, followed by plication, erosion, fissuring and eruption. Again we have an invasion of the sea, which remains until 10,000 feet of shales, sandstones and limestones are deposited; and this is followed by a dry-land period, marked in some places by at least 10,000 feet of erosion and accompanied by plication, fissuring and eruption.

POWELL,69 in 1875, further describes the Grand canyon group. Unconformably below the Carboniferous of the Kaibab plateau is a middle series of slates, sandstones and limestones 500 feet thick, so inclined that the total thickness of its beds is 10,000 feet. Below these are unconformably a thousand feet of crystalline schists with dikes of greenstone and beds of granite. This lower series is composed chiefly of metamorphosed sandstones and shales, which have been folded so many times, squeezed and heated, that their original structure as sandstones and shales is greatly obscured or entirely destroyed, so that they are metamorphic crystalline schists. After these beds were deposited, folded and deeply eroded they were fractured, and through the fissures came floods of molten granite, which now stands in dikes or lies in beds, and the metamorphosed sandstones and shales, with the beds of granite, present evidence of erosion subsequent to the periods just mentioned, yet antedating the deposition of the nonconformable sandstones. Here, then, we have evidences of another and more ancient period of erosion or dry land. Three times has this great region been left high and dry by the ever-shifting sea; three times have the rocks been fractured and faulted; three times have floods of lava been poured up through the crevices, and three times have the clouds gathered over the rocks and carved out valleys with their storms. The first time was after the deposition of the schists; the second was after the deposition of the red sandstones; the third time is the present time.

GILBERT, 36 in 1875, describes the axis of the Black and Colorado mountains in northwestern Arizona as consisting of granitoid rocks and highly crystalline schists. In Bowlder canyon of this range upon a nucleus of syenite are plicated crystalline schists. In Virgin canyon the nucleus is gneissic, with a general anticlinal structure. In Black canyon the nucleus is a homogeneous rock resembling pegmatite, but is probably metamorphic.

In the Grand canyon of the Colorado the Tonto sandstone rests directly on plicated and eroded schists and associated granites, and demonstrates them to be pre-Silurian. Following down the river the same relation is