to be nearly equal, 20°-22°. According to this measurement the prism must have approximately the position of (940); the angle of the latter upon (110) is 21° 2', the angle upon (100) = 23° 58'. A particular phenomenon in the growth of the melilite is the fact that the base does not generally present an even plane, but shows a conical depression. The shape of the lath-shaped sections then resembles the profile of a biconcave lens. Sections parallel to the base are isotropic between crossed nicols and show, when they are not too thin, an indistinct dark cross in convergent light. The cleavage parallel to (001), the cross-fibration of the lathshaped sections and the occurrence of the spear-shaped and pegshaped inclusions arranged parallel to the c axis (the so-called Pflockstruktur) are very distinct. Inclusions of pyroxene, magnetite and glass are common; as already mentioned, these inclusions are generally arranged in zones. In sections parallel to (001) they fill the central parts of their host, and often make up two or three concentric zones. These sections closely resemble leucite because of their rounded shape, the arrangement of the inclusions and the lack of double refraction. Melilite becomes nearly colorless and transparent, but in comparing it with the white, colorless nepheline, it shows a feeble yellow tint. Decomposition has taken place to only a small extent; it begins along the cross-fibration, and greenish-yellow alterationproducts result, the fibres of which are perpendicular to the length of the lath-shaped sections.

Nepheline is always fresh, colorless and transparent; it rarely exhibits a regular shape, but generally forms an aggregate of irregular grains, cementing the other components; it is evidently the latest formed mineral in the rock.

There is abundant magnetite besides perovskite, the common associate of melilite, which occurs in small octahedrons and irregular grains. The perovskite becomes transparent with a brownish-violet color, and shows in some sections a feeble, abnormal double refraction. There appears to be no isotropic base in the normal rock, but if any is present, it must be in a very small amount. There are coarser grained spots in the rock,

which are rich in a partly chloritized base, and in which nepheline occurs in well-shaped crystals.

The second group of rocks, as already mentioned, falls under the head of nepheline-basanite poor in olivine. And since the specimens bear sanidine phenocrysts beside plagioclase, it forms a transition to phonolite. The rock-specimens have a more andesitic than basaltic appearance. Numerous phenocrysts of hornblende and augite are imbedded in the dense bluishgray groundmass. The next most abundant mineral is nepheline in the form of phenocrysts, in part well-shaped crystals, in part rounded, the largest of which are 0.5 cm. in diameter. The nepheline differs from the feldspar in having a grayish color and greasy lustre. Phenocryst of feldspar and crystals of olivine are scarce. Beside these components, the rocks contain apatite, some titanite and iron ores. Under the microscope olivine is seen to be scarce. It is fresh and shows the normal properties. It contains minute octahedrons of picotite and in some sections abundant inclusions of a liquid with moving bubbles.

The amphibole mineral is a typical basaltic hornblende. It becomes transparent with a dark reddish-brown color and exhibits. a strong pleochroism according to the following scheme:

a, brownish yellow, b and dark reddish brown. Absorption, c>b>a.

3

The angle of extinction was examined in sections cut approximately parallel to the clinopinacoid (010) and was determined to be very small. This fact and the dark reddish-brown color are in all probability due to a high amount of Fe2O3. The dependence of the angle of extinction upon the amount of Fe,O, in minerals of the amphibole group has been recently established by Schneider and Belowsky. The basaltic hornblende shows the wellknown dark borders produced by reabsorption by the magma in an early stage of consolidation. In many cases nothing of the original mineral is preserved; the whole hornblende is replaced by a fine grained aggregate of pyroxene and magnetite, presenting clearly the outlines of the absorbed mineral.

The group of pyroxenic minerals is represented by two mono

clinic augites. One of them exhibits a violet-gray color in thin section and belongs to the basaltic augites; the other one becomes transparent with a dark green color. Both form numerous phenocrysts, but the first occurs somewhat more frequently. They occur as single crystals and are also grown together in a zonal manner, the green one always forming the center, the gray one the outer parts of the crystals. Hence the gray augite is the younger. The pyroxene in the groundmass shows the same color and properties. The pleochroism of the two minerals is as follows:

Gray augite.
a Brownish-yellow

b

Violet-gray

Green augite.

Light yellowish-green
Dark gray-green.
Dark green.

The angle of extinction, c: c, is large and, as may be seen in the zonal crystals, it is somewhat larger in the gray pyroxene than in the green. The extinction in sections cut approximately parallel to (010) has been observed to be about 47 degrees (gray augite) and 41 degrees (green augite). The two pyroxenes show in addition to the cleavage parallel to (110) another but less distinct one parallel to (010). Inclusions of magnetite, apatite and glass are common.

Phenocrysts of feldspar are scarce. In part they show the polysynthetic twinning lamination of plagioclase; in part the latter is wanting and one of the latter feldspars, which was isolated and examined for specific gravity and optical properties, was found to be sanidine. Phenocrysts of nepheline are more frequent than those of feldspar. The mineral appears partly in the form of short-prismatic crystals, partly in rounded grains. It presents distinct cleavage, parallel to (1010) and to (0001), and the usually observed optical properties. Isolated grains are decomposed by hydrochloric acid with the separation of gelatinous silica; the resulting solution when evaporated gives numerous cubes of NaCl. Inclusions are scarce; there are fluid cavities with moving bubbles, generally arranged in rows, besides some pyroxene crystals. Apatite forms short and stout crystals always filled with in

clusions of liquids. The opaque ore grains, judging by their ready solubility, belong to magnetite. The groundmass of these rocks consists essentially of pyroxene in well-shaped prisms, lathshaped feldspar, without twinning lamination or in single twins according to the Carlsbad law and nepheline. The feldspar of the groundmass in all probability is mostly sanidine. Nepheline is abundant and occurs in well-shaped crystals. Small patches of a colorless base occur between the crystalline components.

The structure of the rocks is hypocrystalline - porphyritic on account of the occurrence of an isotropic base and the repetition of the crystallization of pyroxene, nepheline and feldspar. Although the specimens by their whole habit and structure belong under the head of nepheline-basanite poor in olivine, the presence of sanidine as phenocrysts causes them to form a transition to the group of phonolites. Unfortunately, analyses of these rocks have not yet been made.

A microscopical examination of the basaltic rock from Pilot Knob, near Austin, Travis County, was made for the purpose of comparison with the rocks from southern Texas just described. The rock was found to be a nepheline-basalt porphyritic with numerous phenocrysts of olivine. The fine grained groundmass consists essentially of augite-crystals cemented by nonindividualized nepheline in very small amount.

A. OSANN.

SOME DYNAMIC PHENOMENA SHOWN BY THE

BARABOO QUARTZITE RANGES OF

CENTRAL WISCONSIN.

THE quartzite ranges of Baraboo extend east and west for about thirty miles, one lying north, and the other, the main range, lying south of the City of Baraboo. The geology of this district is admirably given by the late Professor Irving.' Not only is the general geology clearly described, but remarkably accurate descriptions are given of the character of the quartzite, and the phenomena shown by it, considering the fact that the report was written nearly twenty years since. The unconformity existing between the quartzite and the Cambrian was later more fully described.2 The induration of the Baraboo quartzite has been explained as due to the enlargement of the original quartz grains; and to the deposition of independent interstitial quartz.3 The present note is based upon recent observations on the East Bluff at Devil's Lake and on the exposures at the Upper Narrows of the Baraboo River.

The section across the ranges, as given by Irving, is shown by Fig. 1. The two ranges together, as thus represented, are less than the north half of a great anticline, the south side of the south range being near its crown. This structure involves a very great thickness of quartzite, and was offered with reservation by Professor Irving. He says: "The hypothesis is not altogether satisfactory. The entire disappearance of the other side of the great arch, as well as the peculiar ways in which the

The Baraboo Quartzite Ranges, by R. D. Irving. In Vol. II, Geol. of Wis., pp. 504-519.

2 The Classification of the Early Cambrian and pre-Cambrian Formations, R. D. Irving. In 7th Annual Rep., U. S. G. S., pp. 403-408.

3 Enlargement of Quartz Fragments and Genesis of Quartzites, by R. D. Irving and C. R. Van Hise. In Bull. 8, U. S. G. S., pp. 33, 34.