pebbles. In a little cavity, a few inches in diameter, worn in the floor by the constant and rapid dropping of water from the ceiling, are a number of little pebbles, many of which have been worn round by constant attrition. When this natural mineralogical cabinet was illuminated by magnesium light the effect was very beautiful and brilliant.

- On the Trachyte of Marblehead Neck. By M. E. WADSWORTH, Ph. D. Assistant in the Museum of Comparative Zoology.

Near Boden's Point, on the northwest shore of Marblehead Neck, there is to be seen, exposed between high and low tide, the remains of a trachytic lava flow. This trachyte has flowed over the banded felsite in that vicinity, filling the depressions existing in it at that time, the felsite then as now forming the shore line.

By the alternate action of the sun, rain, and waves, the trachyte has been so destroyed that comparatively little now remains. On the underlying felsite, from one to six inches of felsite débris had been deposited in various places, and over this débris the trachyte flowed, thus burying and preserving it. However, in places the trachyte lies directly upon the felsite without any intervening detritus. The line separating the trachyte from the felsitic débris is well marked, as can be seen in specimens collected by the writer, now in the Lithological Collection at the Museum of Comparative Zoology. This well-marked distinction with the evident line of contact is of especial importance, since it has been claimed that the trachyte was formed out of and is continuous with the felsitic débris. The trachyte is a fine-grained, somewhat crystalline rock, holding here and there porphyritically enclosed sanidin crystals. The color of the rock is, in the least altered portions, a pale greenish, brownish, or purplish gray, weathering to varying dirty yellowish gray and cream-colored tints. On a fresh fracture the least-weathered specimens closely resemble some magnesian limestones (dolomites). Microscopically the groundmass is seen to be composed of a dirty yellowish-white base, holding numerous minute sanidin crystals. The cabinet specimens show under a lens these minute feldspar crystals arranged in fluidal lines, these lines undulating with the surface on which the trachyte rests, and bending around the larger porphyritically enclosed crystals. Laminae of black mica (biotite) were observed in the hand specimens, while microscopically some secondary quartz arising from the decomposition of the groundmass was observed.

On weathering, the rock becomes much jointed, breaking up into thin plates, which platy structure has apparently been taken for stratification.

The writer has taken great pains in his field examination of this trachyte, spending many hours upon it in digging and exposing fresh surfaces in order that its origin and its true relations to the felsite and felsitic débris should be ascertained.

Mr. W. O. Crosby, Instructor in Lithology and Geology in the Massachusetts Institute of Technology, mistook this rock for a "sandstone, or arenaceous slate," which he regarded as Carboniferous, but later as Primordial. He held that the rock was chiefly composed of the felsitic débris, deriving that conclusion from the conglomerate at the base of the trachyte, from which it is entirely distinct. From mistaking the nature and origin of the rock, Mr. Crosby built up quite an extended theory of the formation of Marblehead Harbor, its filling with this "sandstone," and its recent removal, probably by ice (Glacial epoch). Those inter

ested in such matters will find Mr. Crosby's views presented in the following publications: Report on the Geological Map of Massachusetts, 1876, p. 42; American Naturalist, 1877, xi., 584, 585; American Journal of Science, 1880 (3), xix., 122; Contributions to the Geology of Eastern Massachusetts, 1880, pp. 263-265.

It is well known to those interested in the geology of Eastern Massachusetts that the so-called Norian or Naugus Head series was established and held to be the oldest (sic) series of rocks in this district, partly because it was said to send intrusive masses into all the other formations, but was itself cut by none of them. It may then be interesting to note, in passing, that the writer during the past summer found on Marblehead not only granite, but also felsite, cutting the "Naugus Head" series. A well-marked felsite dike can be seen on the beach, just west of Dr. W. F. Whitney's boatlanding, between Fluent's and Dolliber's Points. While the writer does not assent to the views which led to the establishment of the "Naugus Head" series, he points to the above facts in order that they may be explained by those who believe in that series, since these facts are diametrically opposed to their statements and theories.

The Geology and Paleontology of the Tertiary Lakebasin at Florissant, Colorado, are treated in a paper by Mr. S. H. Scudder, Assistant Librarian, in the last Bulletin of Hayden's Survey. The deposits are found about the muchindented shores of an ancient lake-bed, which is about sixteen kilometers long, and lies at an elevation of 2500 meters above the sea, about midway between Colorado Springs and South Park. This lake-bed is surrounded by granitic hills from one to four hundred meters high.

In tertiary times the lake probably emptied into the Arkansas through a small stream at its southern extremity; but the elevation of the district at its outlet drained it in the opposite direction, and the waters of the eroded basin now find their way into the Platte. The region was then the seat of considerable volcanic activity, for the shales which compose the successive floors of the ancient lake to the depth of fifteen meters or more are shown by the microscopic examination of Dr. Wadsworth to be wholly composed of volcanic ash.

The lower beds, forming the present floor of the basin, have yielded no fossils; but the fine-grained ashen shales, which may be found anywhere upon the sides of the low hills enclosing it, preserve a prodigious number of plants and animals, and particularly of insects. Nearly one hundred species of plants have already been recognized (one represented by at least two thousand specimens); while more than fifteen thousand insects, including upwards of a thousand species, have been taken from these beds, showing how much more prolific they are than the long-known European deposits of Oeningen, Aix, and Radoboj. Hymenoptera, Diptera, Coleoptera, and Hemiptera are especially abundant, and even several well preserved butterflies have been found. The examination of this extensive series has only gone far enough to indicate a subtropical fauna, probably more closely allied to that of Radoboj than to others known, but in certain features very distinct. A brief review of the whole is given.

A few fishes, one or two mollusks, and three birds have been found in the same beds, which probably belong to the oligocene, and are considered by Cope to be of the same age as those of Elko, Nevada. A map of the basin accompanies the paper.