MINERALOGY AND PETROGRAPHY.1 Petrographical News.-In an argumentative article on the individuality of rocks Lang2 proposes to regard as a rock individual the product of a continuous (uninterrupted) rock-forming process. Accordingly he would class interbedded clays, sandstones, and conglomerates deposited from the same body of water by a gradual lessening of its velocity, as a single rock, with clayey, sandy, and conglomeratic facies. In the eruptive group, that is a rock individual which has been forced from the depths of the earth by a single earth-throe. The beginning and end of a rock-forming process thus marks the limitations of a rock individual, even though a second period of similar processes should subsequently give rise to a rock of the same nature as that previously formed. The ideas discussed in the article are of great interest, but the practical advantages to be gained by regarding rocks from the point of view of its author are not immediately perceptible.Another article of some theoretical interest is Justus Roth's criticism of Rosenbusch's recent paper on the chemical nature of eruptive rocks. After giving a short historical review of the attempt to explain the variation in eruptive rocks upon a chemical basis, the author proceeds to examine critically the views set forth by Rosenbusch. In the first place, he states that ten of the sixty-three rocks whose analyses are quoted by this writer are much altered from their original condition; others are but local in distribution, while of others, again, the analyses are incomplete. He further continues by showing that the fundamental magmas by whose mixture the various types of rocks are regarded as made up, far from giving rise to definite varieties when mingled in definite proportions, may themselves consist of different combinations of different minerals. He then calls attention to what appear to be weak points in Rosenbusch's calculations, and concludes with the statement that there is not yet sufficient knowledge concerning the chemical character of eruptives to warrant the construction of a theory concerning their composition.Loewinson-Lessing" has attacked the chemistry of the eruptive rocks in a little different way from Rosenbusch. He compares the relations between the bases and the acid in a rock, and calls 'Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 2 Miner u. Petrog. Mitth., XI., p. 467. 3 AMERICAN NATURALIST, 1890, p. 1071. Zeits. d. deuts. geol. Ges., 1891, p. I. 5 Bull. Soc. Belge de Géol., etc., IV., 1890, p. I. 4 MINERALOGY AND PETROGRAPHY.1 Petrographical News.-In an argumentative article on the individuality of rocks Lang' proposes to regard as a rock individual the product of a continuous (uninterrupted) rock-forming process. Accordingly he would class interbedded clays, sandstones, and conglomerates deposited from the same body of water by a gradual lessening of its velocity, as a single rock, with clayey, sandy, and conglomeratic facies. In the eruptive group, that is a rock individual which has been forced from the depths of the earth by a single earth-throe. The beginning and end of a rock-forming process thus marks the limitations of a rock individual, even though a second period of similar processes should subsequently give rise to a rock of the same nature as that previously formed. The ideas discussed in the article are of great interest, but the practical advantages to be gained by regarding rocks from the point of view of its author are not immediately perceptible.Another article of some theoretical interest is Justus Roth's criticism of Rosenbusch's recent paper on the chemical nature of eruptive rocks. After giving a short historical review of the attempt to explain the variation in eruptive rocks upon a chemical basis, the author' proceeds to examine critically the views set forth by Rosenbusch. In the first place, he states that ten of the sixty-three rocks whose analyses are quoted by this writer are much altered from their original condition; others are but local in distribution, while of others, again, the analyses are incomplete. He further continues by showing that the fundamental magmas by whose mixture the various types of rocks are regarded as made up, far from giving rise to definite varieties when mingled in definite proportions, may themselves consist of different combinations of different minerals. He then calls attention to what appear to be weak points in Rosenbusch's calculations, and concludes with the statement that there is not yet sufficient knowledge concerning the chemical character of eruptives to warrant the construction of a theory concerning their composition. Loewinson-Lessing" has attacked the chemistry of the eruptive rocks in a little different way from Rosenbusch. He compares the relations between the bases and the acid in a rock, and calls Edited by Dr. W. S. Bayley, Colby University, Waterville, Maine. 2 Miner u. Petrog. Mitth., XI., p. 467. 3 AMERICAN NATURALIST, 1890, p. 1071. Zeits. d. deuts. geol. Ges., 1891, p. I. 5 Bull. Soc. Belge de Géol., etc., IV., 1890, p. I. = those rocks acid that contain an excess of the latter,―i. e., more than enough to saturate the bases present. This excess shows itself as free quartz. Thus it is not the percentage of silica in a rock-mass that carries it into the acid, neutral or basic class, but it is the lack or excess of silica as compared with the bases. Several empirical formulas represent the author's types. They are based on the determined ratio between bases and acid in neutral rocks, in which the amount of silica necessary to saturate the bases (in percentage) is represented by formula SiO,= 2(R,O+RO) +R,O,. In neutral rocks this relation exists, consequently the formula represents the composition of such rocks. The amount of silica in basic rocks is represented by SiO,⇒R2O+RO+R,O,, and in acid rocks SiO, 2(R2O+RO)+R,O,+Quartz,―i. e., the percentage of silica in these is greater than twice the sum of the percentages of R,O and RO, plus R,O,, by as much as there is quartz in the rock. The author then uses these formulas, with others deduced from them, as a basis for the classication of eruptives. His paper is interesting reading, and if facts justify his formulas the conclusions reached by the author will prove of value to petrographers. Some recent discussions on the pressure alteration of basic eruptives have added quite a little to our information regarding this phenomenon. Welch has described the alteration of diabases into schists in the region of the Soonwalde, in the Taunus, on the left side of the Rhine. Well-developed diabases exist near Rauenthal. Under the influence of pressure they have given rise to schistose rocks containing actinolite, and others in which chlorite and epodite have replaced the original augite. Other schists, whose relationship to diabase can be determined only by a microscopical examination, have been called hornblende-sericite-schists, augiteschists, and sericite-calc-phyllites. The author has made a careful study of all these, which has resulted in their separation into schists composed of actinolite and epidote, those containing a blue amphibole, and finally those made up principally of chlorite. In some of these derived rocks augite may still be detected, in others the diabase structure is still visible, while in a third class no traces of the original constituents nor of their arrangement are recognizable. All show evidence of pressure in the shapes of their components and in their nature. In the first class the epidote and chlorite have been derived from augite. The second class contains, in addition to glaucophane, a bluish-green actinolite, sericite, and biotite. Epidote is entirely wanting. The rocks of the third group are combinations of chlorite, quartz, sericite, and generally calcite. In each the structure is schisZeits. d. deuts. geol. Ges., XLI., 1890, p. 394. tose, and the epidote mos able. Again viz.: the sy between adj the same co absence of i that segreg yielding 66 S tose rock fro with other s schist derive be split in constituents in small tra parallel to t mineral ma other schist of chlorite, rite is prol talc. The suggestion seen betwe Prof. Pump Geol. Mag 8 Ib., Nov., Ib., Dec., Bull. Gec tose, and the mother-rock was a diabase or a diabase-porphyrite. The paper concludes with a discussion of fifteen analyses of the rocks described. Dynamically altered diabase and gabbros (sometimes schis. tose) occur also as sheets and dykes cutting altered sediments at St. Johnstown and Raphoe, in N. W. Ireland. Among them are epidiorites in which, according to Hyland, the hornblende still preserves the ophitic structure of the original augite. The original feldspar was labradorite, but the changes effected in it have yielded a quartz-feldsparepidote mosaic in which the secondary feldspar is oligoclase. The schists, granulites, and even some of the massive rocks of the Lizard, England, are distinctly banded. To account for the phenomenon it has been suggested that it is due either to original sedimentation, or to the deformation of eruptive rock-masses, or to the injection of rock material along planes of weakness in preëxisting rocks. Since the schists are now known to be eruptive, the first explanation is not available. Against the second Somervail brings the following objections, viz. the symmetry of the banded structure, the frequent transitions between adjacent bands, and the uniform banding in large masses of the same composition. The injection theory is contradicted by the absence of irruptive contacts. He accounts for it on the supposition that segregations formed during the cooling of the magmas, yielding "schieren" that were afterwards squeezed.—A green schistose rock from near Zermatt, in the Pennine Alps, occurs so associated with other schists that Bonney is compelled to regard it as a pressure schist derived from serpentine. The rock is so very fissile that it may be split into sheets one-eighth of an inch thick. Only two essential constituents are observed in it, one an olive-green mineral, occurring in small translucent flakes with a cleavage like mica and an extinction parallel to this, and the other a chromite or magnetite. The former mineral may possibly be antigonite. Associated with this rock are two other schists: one, a green schist, is a soft chloritic rock, composed of chlorite, magnetite, and zoisite (?). The composition of the chlorite is probably near that of chloritoid. The other schist is full of talc. The origin of neither of these could be determined. ————A suggestion in explanation of the cause of the transitions sometimes seen between crystalline and clastic rocks has recently been offered by Prof. Pumpelly, 10 who believes that rock disintegration by weathering Geol. Magazine, 1890, p. 205. 8 Ib., Nov., 1890, p. 509. Ib., Dec., 1890, p. 533. Bull. Geol. Soc. Amer., Vol. II,, p. 209. 4775 |