ON THE AGE OF THE COPPER-BEARING ROCKS OF LAKE Superior. By M. E. WADSWORTH, of Cambridge, Mass. [ABSTRACT.]

Two principal views regarding the relative age of the copperbearing traps of Keweenaw Point and the eastern sandstone have been and are now held. The first regards the traps and sandstone as of the same age; the second places the traps as the earlier formation (pre-Paleozoic), and the eastern sandstone as the later one (Paleozoic). The traps are said to have formed a sea wall along the eastern side of the Point, while the sandstone was deposited against the base of the cliff. The sandstone is said to be horizontal, or at most to have only a slight inclination at the base of the cliffs, at which point it contained pebbles of the trap derived from them. This evidence was principally obtained at the Douglass Houghton Falls, and on the face of the published accounts appears conclusive. It has given rise to the supposition that the traps formed a distinct geological series. belonging to Azoic or Archean time, which series has been recognized by lithological characters far and wide. As this Keweenawan series was first established on Keweenaw point, it must stand or fall by observations made there, and not, as some would make it, by observations in Wisconsin, the northern shore of Lake Superior, or in Newfoundland.

By personal observations at the Douglass Houghton Falls, I have found that the eastern sandstone instead of being horizontal as stated, dips north 45° west, 25°, while passing down the river the dip gradually diminishes in angle until it is only 5° at the mouth of the ravine below the falls. Instead of the cliffs at the falls representing the termination of the copper-bearing traps as stated, I found several basaltic overflows below the falls interstratified with the conglomerate and sandstone. The relation of the copper-bearing traps below the falls to the interbedded sandstones and conglomerates, is the same as it is in the rocks above the falls. The last basaltic flow here is a thin sheet some two feet in thickness, which is interbedded between sandstones, having the same dip as the basalt. The finding of the copper-bearing rocks below the Douglass Houghton Falls, explains the presence of pebbles of melaphyr in the conglomerate at the falls, and shows that the observations referred to were made within the region of the copper-bearing traps, the observers not having found the junction of

the traps with the sandstone at all. Something more is necessary in such observations than simply to find a sandstone on the eastern side of the copper-bearing rocks; it is necessary to know that this is part of the eastern sandstone and not a bed intercalated with the trap. Now as I find that the eastern sandstone underlies the trap conformably, that is, as conformably as a bed can underlie a lava which has flowed over it, it must be older in order of time, but of the same geological age with the copper-bearing rocks. As my observations show the incompleteness of those on which the "Keweenawan series " was founded, it seems right to hold, until my observations shall be disproved, that the "Keweenawan series" has no foundation, but that the copper-bearing rocks are of the same age as the eastern sandstone. This series has been advocated upon lithological evidence, and said to be recognized in other localities from this evidence, but the above observations furnish another proof of the absurdity of founding and identifying geological systems simply by lithological characteristics.

In ascending the Hungarian river, I found the same relations of the trap to the sandstone. The last bed of the eastern sandstone, dipping north 45° west, 20°, is overflowed by the first bed of trap which has baked the former; the same as these basaltic flows indurate the sandstone within the copper-bearing rocks. This thin flow is overlaid by a conglomerate and sandstone, at the base of which we find pebbles of the underlying trap. Furthermore, similar alternations of trap, sandstone, and conglomerate extend until the western sandstone is reached.

It seems then that the writer has advanced sufficient evidence to prove that in the parts visited, the eastern sandstone conformably underlies the copper-bearing rocks and that both are of the same geological age.

The question of the relative ages of these rocks was the one with which my time was occupied, but so far as the absolute geological age of the sandstone, and therefore of the trap, is concerned, it would seem that the evidence brought forward by Dr. Rominger, until disproved, shows that Messrs. Foster and Whitney were correct in regarding it as of Potsdam age.

For a fuller treatment of this subject the reader is referred to another publication.1

1 Notes on the Geology of the Iron and Copper Districts of Lake Superior. Bulletin of the Museum of Comparative Zoology, 1880, Vol. VII (Geol. Series, Vol. I), No. 1, 157 pp., 6 plates.

TURQUOISE OF NEW MEXICO. By B. SILLIMAN, of New Haven, Conn.

THE existence of this comparatively rare gem in New Mexico is a fact long known-the chief locality being at Mt. Chalchuitl in Los Cerillos, about twenty-two miles southwest of the ancient town of Santa Fé, the capital of that territory. We are indebted to Prof. Wm. P. Blake for our first detailed notice of this ancient mine in an article published in 1857, in the American Journal of Science.1

It was subsequently visited by Dr. Newberry and mentioned in one of his reports, and by others. I have lately had an opportunity of examining this very interesting locality, since it has been laid open in the old workings, by the recent explorations of Mr. D. C. Hyde, and thus rendered accessible to observation.

The Cerillos Mountains have recently come into notice from the partial and as yet, superficial exploration, of very numerous mineral veins which are found to intersect them and which carry chiefly argentiferous galena, with some gray copper, rich in silver, giving promise of mines of value when opened in depth. I have elsewhere spoken more particularly of these veins and of the rocks that contain them. These rocks are all eruptive or plutonic rocks of the family of the Augite Trachytes, the microscopic study of which reveals the interesting fact that they are of the same family which, the world over, carry the richest and most permanent ores of silver, with some gold, and which there is good reason to believe are here penetrated by true fissures which may be followed to any depth, without exhaustion. In the centre of this district, which is not more than about six miles by four in extent, rises the dome of Mount Chalchuitl, the summit of which is about 7,000 feet above tide, and is therefore almost exactly on a level with the Plazza of Santa Fé, across the valley of the river of that name, to the northeast. In the other direction this mountain has its drainage into the valley of the Galisteo, which forms the southern boundary of the Cerillos district. The geological age of the eruption of these volcanic rocks is probably Tertiary. The rocks which form Mt. Chalchuitl-the Indian name of the turquoise-are at once distinguished from those of the surrounding

[II.] XXV. 27.

and associated ranges of the Cerillos by their white color and decomposed appearance, closely resembling tuff and kaolin, and giving evidence, to the observer familiar with such phenomena, of an extensive and profound alteration, due, probably, to the escape through them, at this point, of heated vapor of water and perhaps of other vapors or gases, by the action of which the original crystalline structure of the mass has been completely decomposed or metamorphosed, with the production of new chemical compounds. Among these the turquoise is the most conspicuous and important. In the seams and cavities of this yellowish-white and Kaolin-like tuffaceous rock the turquoise is found in thin veinlets and little balls or concretions called "nuggets," covered on the exterior with a crust of the nearly white tuff, and showing on cross fracture the less valued varieties of this gem, more rarely offering fine sky-blue stones of higher value for ornamental purposes. It is easy to see these blue-green stains in every direction among these decomposed rocks, but the turquoise in masses of any commercial value is extremely rare, and many tons of the rock may be broken without finding a single stone which a jeweller, or virtuoso, would value as a gem.

The observer is deeply impressed on inspecting this locality with the enormous amount of labor which in ancient times has been expended here. The waste or débris excavated in the former workings covers an area which the local surveyor assured me, by his measurement, extends over at least twenty acres of ground, and which it is easy to see has a very large bulk. On the slopes and sides of these piles of rubbish are growing large cedars and pines, the age of which-judging from their size and slow growth in this very dry region-must be counted by centuries. It is well known. that in 1680 a large section of the mountain suddenly fell in from the undermining of the mass by the Indian miners, killing a considerable number, and that this accident was the immediate cause of the uprising of the Pueblos and the expulsion of the Spaniards, which happened in that year just two centuries since.

The accompanying vertical section of the mountain from east to west will give a good idea of the old workings and of the shafts and tunnels projected and partly carried out, by Mr. Hyde. The irregular openings, named by Mr. H., "wonder caves" and the "mystery," are the work of the old miners, and the whole hillside from the flag-staff to the "mystery" was worked out by them also.

color so familiar in the Mexican pottery, and associated with these, numerous stone hammers, some to be held in the hand and others swung as sledges, fashioned with wedge-shaped edges and a groove for the handle. In one case, a hammer weighing over

It was this sharp slope of the mountain which fell. In these chambers, which have some extent of ramification, were found abundantly the fragments of their ancient pottery, with some entire vessels, some of curious workmanship, ornamented in the style of