face brightness, their ellipticity of figure, difference in brightness of opposite sides of the stars, and the eccentricity of the orbit in which they revolve. If the components, moving in elliptical orbits, are not spheres, a slow progressive change in the direction of the longest axis of the relative orbit is to be expected. Such a perturbation would cause the interval between corresponding eclipses to vary in a periodic manner, for a while coming later than the times predicted by a constant period, and again, after several years, later. Stars of the Cepheid type increase rapidly in brightness to a maximum and then fade out gradually, to repeat the performance after a certain nearly constant interval. The variation of the Cepheid type is probably not due to the eclipses of a binary system but possibly rather to periodic pulsations within the star. The effects upon the brightness of the star are, as might be expected, less simple and regular than is the case with Algol variables. The periods of Cepheid variables are found to vary slowly, and there is also a more or less periodic change in the rapidity and manner of the light variation. The spectral type or color of several stars of both types is different at minimum and maximum brightness. This shows that the components of an eclipsing system are

often not of the same color and are at different stages of evolution, and that in the Cepheid variables constitutional changes accompany the light variations. The spectra of the latter change periodically along the normal stellar spectral series. (Astron. Nachr., ccii, 137; Astrophys. Jour., xliii, 217; xliv, 51; Proc. Natl. Acad. of Sciences, April, 1916; Monthly Notices Roy. Astron. Soc.)

Proper Motion.-The slow motion of the stars on the background of the sky-motion at right angles to our line of sight, the so-called proper motion, which can only be detected by comparison of measurements made several years apart, leads to many interesting inferences concerning the structure and history of our universe. A star of the 10th magnitude is found by Barnard to have the largest proper motion yet known, amounting to 10".3 a year. The star is moving almost due north. The rapidity with which it moves among the other stars is largely due to its unusually small distance from the solar system. Preliminary calculations give it a probable parallax of considerably more than half a second. Since the star is both near and faint, its absolute brightness is very small-less than one three-thousandth that of the sun. This star is in fact the faintest known. (Astron. Jour., xxix, 181.)

GEOLOGY

J. B. WOODWORTH Pre-Cambrian and Cambrian.-Andrew C. Lawson (Bull. Dept. Geol. Univ. Cal., x, 1-19, 1916) correlates the Pre-Cambrian rocks of the Great Lakes region on the hypothesis of two periods of granitic intrusion. In the author's scheme, Pre-Cambrian, following the usage of Van Hise, includes an older, Archean, and a younger, Algonkian, division. The Archean embraces an older Ontarian Period or Series, divided from below upward into the Coutchiching, the Keewatin, and the Grenville Epoch or Series, followed by the first or Laurentian epoch of great granitic intrusions. The Epilaurentian interval of erosion, marked by a major unconformity, separates the older Ontarian from the younger Archean or Huronian Period or System. The Huronian consists of the Bruce Epoch or Series, to which succeeds, generally in unconformable superposition, the Temiskamian Epoch or Series. This record is closed by the Algoman episode of granitic intrusions, followed by widespread erosion giving rise to the unconformity between the above named oldest rocks of the Archean and the overlying Algonkian. To this erosion interval the name Eparchean is applied. The formations immediately succeeding the unconformity are doubtfully referred by Lawson to the Paleozoic era, under the caption Algonkian Period or System, of which the Animikian Epoch or Series is the lower, and the Keweenawan Epoch or Series the upper member.

The Pre-Cambrian rocks of the Canton quadrangle in the Adirondacks are reported on by James C. Martin (N. Y. State Museum Bull. 185, 1916);

of the folded and contorted structure of schistose rocks. W. J. Miller describes (ibid., No. 182, 1916) the geology of the Lake Pleasant quadrangle, a region of Palæozoic strata locally downfaulted in areas of PreCambrian. The relations between the Pre-Cambrian and Cambrian formations in the vicinity of Helena, Mont., are described by C. D. Walcott (Smithson. Misc. Coll., lxiv, 1916, 259301) in refutation of the arguments of Dr. August Rothpletz (Die Fauna der Beltformation bei Helena in Montana, Munich, 1915). L. F. Noble and J. F. Hunter (U. S. Geol. Surv., Prof. Paper 98-I) find that the Archean complex of the Granite Gorge in the Grand Cañon of Arizona includes under the name Vishnu schist two or more very different series of metamorphic rocks of igneous and sedimentary origin.

Folds and Faults.-R. A. Daly (Bull. Geol. Soc. Amer., xxvii, 1916, 89-92) proposes the adoption of the term "homocline" to include "any block or mass of bedded rocks all dipping in the same direction." The term monocline he would restrict to its onetime usage as found in the writings of Dutton and Sir Archibald Geikie, in which sense it is the English equivalent of flexure as advocated by Heim and De Margerie. The article sets forth the different uses of the term monocline. Winthrop P. Haynes (Jour. Geol., xxiv, 1916, 269-290) gives an account of the rocks involved in the Lambert overthrust near Three Forks, Montana. The mechanics of the Panama Canal slides are treated mathematically by G. F. Becker (U. S. Geol. Surv., Prof. Paper 98-N, pp. 253-261) in a paper of general application to the understanding of the formation and movement of landslides

Palæozoic Banded Shales.-Several geologists have proposed to account for the alternating bands of thick and thin, fat and lean (or sandy) clays, in the recent glacial series by the waxing and waning of clay-depositing waters dependent upon the rotation of the seasons during an epoch of glaciation, one layer being referred to summer or the period of maximum melting of the ice and consequent increased deposition, the other to winter with its diminished outwash of detritus. R. W. Sayles finds that the banded shales and slates at Squantum, Mass., associated with the tillite or glacial boulder bed of that locality, are essentially like the modern glacial clays in structure and origin. He therefore concludes that during the Permian glacial epoch there were seasonal changes involving the recognition of a winter and summer (Proc. Nat. Acad. Sci., ii, 1916, 167-170).

Mud Lumps.-The mud lumps of the Mississippi delta have long proved troublesome to engineers and have been little understood by geologists. E. W. Shaw (U. S. Geol. Surv., Prof. Paper 85-B, 1913) discusses their now known structure and probable origin. He ascribes the formation of the mud lumps to a bodily flowage of the delta towards its unsupported edge, resulting in a squeezing of the soft layers and the accumulation of the clays in places where the pressure is less. This explanation accords with many previous observations on dikes and irregular intrusions of clays in recent delta deposits in regions recently uplifted from the

sea.

of Arizona are summarised in a paper by F. L. Ransome (U. S. Geol. Surv., Prof. Paper 98-K, 1916, 133166). The Ordovician is locally identified in the region, but Silurian strata have not been recognized. In the Gulf coastal plain, Matson and Berry have worked out and described the Pliocene as a single formation of chiefly non-marine origin known as the Citronelle, varying from 50 to 400 ft. in thickness. From eastern Texas to Alabama the formation occurs in high level terraces mainly composed of yellow and red sands and clay. The formation is represented by marine marls in peninsular Florida.

The "Upper Cretaceous Deposits of Maryland" is the title of the sixth volume of a series of reports on the geology of that state. The present work is by the Director, Dr. W. B. Clark. The formations described in ascending order are the Raritan, Magothy, Matawan, Monmouth and Rancocas.

Minor Studies.-C. W. Tomlinson (Jour. Geol., xxiv, 1916, 253) reviews the red-bed question, concluding, as Barrell has stated, that "red color in sediments is not itself an indication of aridity." Donald C. Barton presents a much needed discussion of the definition of the term "arkose" and the interpretation of the deposits of this rock (ibid., 417-449). The types of prismatic structure in igneous rocks are ascribed by R. B. Sosman (ibid., 215-234) to thermal contraction of the rock after crystallization; contraction and separation while the magma is only in part crystalline; and, lastly, internal expansion as shown by "weather cracks" in diabase boulders. R. M. Field has a paper (Ottawa Naturalist, xxx, 1916) on the origin and classification of intraformational conglomerates and breccias.

Geological Surveys.-A geological reconnaissance of Porto Rico by C. P. Berkey (Ann. N. Y. Acad. Sci., xxvi, 1915, 1-70) has resulted in the publication of a black-line geological map of the island showing the approximate boundaries of the Tertiary formation. Two structure sections represent the relations of the highly inclined sediments and the igneous

The summary report of the Geological Survey of Canada for 1915 (Ottawa, 1916) includes in R. M. Anderson's account of the Canadian Arctic Expedition (p. 220) the geological report of J. J. O'Neill (pp. 236-241). Numerous condensed accounts of geological surveys carried on by the staff accompany the report. A. E. Foerste (Memoir 83) describes the Upper Ordovician forma tion in Ontario and Quebec. J. D. Mackenzie (Memoir 88) gives an account of the geology of Krakaur Island, B. C.

Bibliography.-Bulletin 645 of the U. S. Geological Survey includes a list of papers on the geology of North America published in 1915. J. C. Branner's Geologia Elementar (2d ed., Rio de Janeiro, Brazil), a textbook of geology prepared especially for Brazilian students, contains the best compendium of Brazilian geology and also several small scaled geological maps of the country. H. F. Cleland has published Geology: Physical and Historical (New York Am. Book Co., 1916), and Wm. J. Miller, An Introduction to Historical Geology (New York, D. Van Nostrand Co., 1916). Field Geology by Fred H. Lahee (New York, McGraw-Hill Book Co., 1916) is a useful compendium on the lines of Sir A. Geikie's book of 1879.

ECONOMIC GEOLOGY

ADOLPH KNOPF

Secondary Sulphide Enrichment.Progress in the theory of ore deposits during the year has centered mainly on the problems of secondary sulphide enrichment. These problems have recently attracted the attention of able chemists, and in consequence much fruitful experimental work is now being done on them. Since work of this kind should be carried on at ordinary temperatures in order to reproduce the conditions prevailing in nature, the difficulties, already great, are much increased, because of the inertness of the mineral sulphides. Young and Moore, working in the laboratories of Stanford University, believe, however, that eventually all

have developed a procedure that appears destined to throw much light on mineral transformations; they call this the "method of artificial replacement." Minerals after being sectioned and polished may be studied and mapped, then treated with various reagents, and the change noted after a repolishing. Replacements resembling those seen in natural minerals, such as the formation of chalcopyrite at the expense of melaconite veinlets traversing chalcocite, were ef fected in comparatively short time in the laboratory.

Zies, Allen and Merwin, working in the Geophysical Laboratory of the Carnegie Institute, have investigated the action of copper sulphate solutions on the sulphides commonly occurring in copper deposits. These reactions are peculiarly important, inasmuch as it is due to them that the great masses of ore-bearing porphyry, whose output now dominates the copper industry, became sufficiently enriched in copper to be commercially valuable. A number of important equations are quantitatively established. The main geologic result that may be drawn from the immense amount of detailed work done is that chalcocite predominates universally in supergene enriched ore because cuprous sulphide is more insoluble than cupric sulphide and because covellite is oxidized to chalcocite by cupric sulphate.

"The Enrichment of Ore Deposits" by W. H. Emmons (U. S. Geol. Surv. Bull. 625) is an amplification of an earlier bulletin on the enrichment of sulphide ores published in 1913. In the new report all the enrichments effected during oxidation are considered and the reactions of 15 chemical elements not discussed in the earlier report have been added. It is interesting to note that, as pointed out by Emmons, Whitney, writing in 1854, first applied the theory of secondary sulphide enrichment to account for the rich mass of bluish-black sulphide below the gossan at Ducktown, Tenn. Although the fundamental principles of the theory were clearly grasped by Whitney so early, it was not until 1900, after their rediscovery and announcement by S. F. Em