Lossen and Koch also mention cordierite as an abundant constituent in the kersantite of Michaelstein, although this mineral does not appear in the inclusions.* II. Contact-metamorphism in the Limestone. The limestone beds associated with the gneisses and mica schists of Westchester County have been carefully mapped and described by Professor James D. Dana.† They are regarded by him as belonging to the Green Mountain system and as probably of Lower Silurian or Cambrian age. Some of this limestone appears within the limits of Cortlandt township, but it falls within the scope of this paper only in so far as it has been modified by the action of the intrusive rocks of the Cortlandt Series. The western edge of Verplanck Point, which projects into the Hudson River south of Peekskill, is formed of the Tompkins Cove limestone, so admirably exposed in the large quarries on the opposite side of the river. At the southern end of the point the limestone is in contact with the massive rocks, which here display in the most unmistakable manner the evidence of their eruptive character. This fact is admitted by Professor Dana, although he is inclined to regard the dykes there exposed as softened or fused sedimentary material rather than truly exotic intrusives.‡ * At the close of his series of articles on the Westchester County rocks, Professor Dana has very admirably summarized all the essential contact phenomena displayed at Cruger's. (This Journal, III, xxii, p. 314, Oct. 1881.) He here also advances four considerations which he believes to be adverse to the idea that the contact-phenomena were produced by the action of an eruptive rock. These considerations are briefly as follows: 1. The crumpling of the schist must have been produced at the time of its metamorphism. 2. An intruded rock would have been too feeble an agent to produce this. 3. The increase of metamorphism would only have needed an increase in temperature, and this may have been caused by the crumpling of the strata. 4. Staurolite and fibrolite are widely distributed through the mica schists. Evidently the metamorphism of the original rock-whatever its characterinto a mica schist must have taken place at the time of its folding and crumpling. Now it is by no means certain that the forcible intrusion of a large body of molten rock, far below the surface, may not have exerted a pressure in accordance with laws of hydrostatics sufficient to have caused the local puckering to be seen at Cruger's. But this does not necessarily have any bearing on the question. The original rock may have been crumpled and changed to a mica schist by some orographic force, as Professor Dana thinks probable, and subsequently have undergone a further contact-metamorphism by the agency of the Cortlandt eruptives. The intrusion of the massive rock itself may have been-probably was—the result of orographic movements. This intrusion may or may not have caused the puckering, but the progressive metamorphism observed at Cruger's bears such a direct relation to the contact line that the conclusion seems unavoid-. able that at least the mineralogical changes above described are directly due to the influence of the eruptive rocks + On the Geological Relations of the Limestone Belts of Westchester County,. N. Y. This Journal, 1880 and 1881, vols. xx, xxi, xxii. + This Journal, III, xx, pp. 200-203 and 216, Sept., 1880. Near the foot of Broadway in Verplanck the limestone fragments occur imbedded in the norite as figured by Professor Dana,* while just behind and beyond the hotel branching dykes of the massive rocks, varying from a fraction of an inch to many yards in width, penetrate the limestone in different directions, though in the main they follow the direction of its bedding. The microscope shows that these dykes belong to many different rock types, as for instance gabbro (Nos. 109, 111), mica-hornblende-diorite (No. 80), hornblende-diorite (Nos. 82 and 83) and mica-diorite (No. 81). The metamorphic action extends but a small distance from the actual contact, but is always unmistakable in its nature. The limestone is in almost all cases bleached and is frequently rendered more coarsely crystalline. There are new contactminerals developed in it, among the most common of which are hornblende and pyroxene, both rich in lime and of a pale color. In the narrowest dykes, the nature of the eruptive rock is also considerably modified. In one of these (No. 77), the entire width of the dyke (only one-eighth of an inch) is contained within the thin-section. The intrusive material consists of brown hornblende, biotite, triclinic feldspar, apatite and an abundance of a brightly polarizing, uniaxial, negative, colorless mineral, which is probably scapolite. The brown hornblende is concentrated along the edges of the dyke and often stands perpendicular to its walls. The limestone beside the eruptive material is highly crystalline and contains colorless pyroxene and muscovite. In some cases, as for instance in a section of Professor Dana's collection, the metamorphic action is extremely slight even when seen under the microscope. This particular section contains the contact between a mica-hornblende-diorite and limestone. The only change in the former is a concentration of hornblende along the contact line, while the limestone has become slightly more crystalline and contains oval spots of serpentine. In the cutting exposed on the West Shore Railroad at Stony Point, a narrow band of limestone is seen between the masses of mica-diorite and peridotite, which, but for this, come here in contact. This limestone is very crystalline and is filled with minerals doubtless derived from the eruptive rocks. There is a pale pyroxene (malacolite), a light green hornblende, zoisite, sphene, and quite abundant scapolite. At the conclusion of this series of papers, it may be advisable to summarize the evidence in favor of the eruptive (igneous) * This Journal, III, xx, p. 202, fig. 5. origin of the massive members of the Cortlandt Series; this can be done as follows: 1. The general character of the rocks themselves, which, both in structure and mineralogical composition, agree with well recognized eruptive types. 2. The differentiation of massive rocks into facies which occupy irregular areas; i. e. there is nothing in the alternation of the different types to suggest an originally sedimentary structure. 3. Occurrence of the massive rocks in well-defined dikes (a) in other massive rocks. Point. Montrose Point and Stony (b) in mica schist. Stony Point. (c) in limestone. Verplanck Point. 4. Occurrence of angular rock fragments (inclusions) in the massive rocks. (a) of crystalline schist. Cruger's. (b) of limestone. Verplanck Point. 5. Production of contact phenomena in the stratified rocks adjoining the massive ones: a) in crystalline schists. Cruger's. Stony Point. It is probable that the Cortlandt Area was once the scene of prolonged volcanic activity from several vents. There is, however, every reason to suppose that the rocks now exposed solidified at a considerable depth below the surface and have since been brought to light by erosion. This is indicated:1. By the coarse-grained structure of many of the massive rocks. themselves, which gives to them a plutonic rather than a volcanic or superficial character. 2. By the absence of any tuff deposits or topographical resemblance to volcanoes. 3. By the presence of a marked contact zone, such as is only produced around deep-seated eruptive masses, where the enclosed vapors cannot readily escape. The seat of intensest action would seem to have been near the center of Cortlandt township, where norite is the prevailing rock. The most diverse types are peripheral in their distribution, especially toward the west and south. In these quarters also occur frequent remains of the original country rock in the way of bands of limestone or patches of mica schist. These have all suffered more or less intense metamorphism. It also seems probable that the iron and emery beds along the southern and eastern portions of the massive area, are to be regarded as the result of metamorphic action upon preëxistent material. Petrographical Laboratory, Johns Hopkins University, Baltimore, May, 1888. Explanation of figures in Plate VI. Fig. 1. Typical, unaltered mica schist. River bank near Cruger's Station (“1” on the map), No. 21. Section cut approximately parallel to the foliation, shows the biotite mostly in basal sections of varying thickness. Large flakes of muscovite irregularly scattered through the field. Occasional crystals of tourmaline and zircon. Groundmass mostly a granular aggregate of quartz with very little feldspar. (× 30). Fig. 2. Fibrolitic schist, collected a short distance north of the last on the line of Section I (M. of Professor Dana's collection). Fibrolite very abundant in large sheaf-like bundles, and also in radiating tufts, garnet and biotite. Groundmass composed of coarse grains of quartz with some feldspar. (×30). Fig. 3. Staurolitic mica schist ("o" on the map) No. 25. Large crystals of yellow staurolite with irregular boundaries and the usual quartz inclusions at their center scattered through a matrix of quartz and biotite. Some fibrolite in radiating tufts. (× 30). Fig. 4. Fibrolitic schist from very near the contact ("p" on the map) No. 37. Rock very largely composed of fibrolite in feathery tufts, bundles and sheaves. Matrix almost wholly a brown mica. There also occur in lesser quantity cyanite, garnet and staurolite. (× 30). Fig. 5. Black inclusion in the mica-diorite (near "s" on the map) No. 29. Composed mainly of pleonaste in small octahedral crystals among which are scattered larger crystals of colorless corundum. Where the section is sufficiently transparent, feldspar and biotite in small quantity may be recognized. (× 30). Fig. 6. Inclusion in mica-diorite ("s" on the map) No. 12. The most important constituent is margarite in colorless crystals. Other minerals are green mica (ripidolite), biotite, quartz, feldspar aud magnetite. Tourmaline and epidote also occur in these inclusions, although they are not represented in the figure. ( × 30). ART. XXVII.-The Sedentary Habits of Platyceras; by C. R. KEYES. THE genus Platyceras was founded by Conrad* in 1840 for a Paleozoic group of gasteropodous shells "sub-oval or subglobose, with a small spire, the whorls of which are sometimes free and sometimes contiguous; the mouth generally campanulated or expanded." Hitherto these fossil shells had been referred to the genus of modern Mollusca Capulus, proposed by Monfort† in 1810. Conrad's name, however, for this fossil group was not until within the past few years generally accepted, preference having been given by most European writers and also by some American authors to Acroculia of Phillips, notwithstanding the fact that the type of Phillips's genus was a typical form of Platyceras. Although the forms of this genus present so few classificatory characters of definite specific value more than three hundred species have been described-over one-half of which are from America. In this genus, as in many other Paleozoic genera, numerous species have been based not on any apparent distinctive character, but seemingly simply on their occurrence at different geological + Conch. Syst., vol. ii. * Ann. Rep. Palæ. N. Y., p. 205. horizons; and this has given rise to the establishment of many species which are unquestionably invalid. The synonymy of the species of Platyceras when fully worked out will doubtless result in quite an extensive numerical reduction of the species. In the absence of better defined characters for specific distinction considerable importance has been attached to the configuration of the peristome, but even this character in the majority of the species of this genus now appears to have little, if any, classificatory value. A careful comparison of a large series of different species of Platyceras reveals the fact that the apertural margin in various specimens of the same species often presents considerable variation: a phenomenon not to be entirely unexpected in a group so closely allied to the modern Capulus, from which some writers even now question the propriety of generically separating Platyceras. Among the living Calyptræide it has been observed that both color and form are to a great extent dependent upon individual environment, and hence among forms of the same species there may be many varietal phases. It has further been noted that the majority of the members of this family attach themselves, while yet quite young, to stones and shells of other Mollusca and, having once found a suitable situation, seldom, if ever, remove from the spot where they first settled. The character and contour of the surface on which they have settled would therefore determine the form and outline of the apertural margin. The sedentary habits of the modern representatives of this group of Mollusca would be suggestive of analogous habits among their fossil congeners. Notwithstanding the comparative abundance of Platyceras in some of the Paleozoic strata of both this country and Europe direct paleontological evidence of the sedentary habits of this group is not often met with; yet the instances presented, independent of their bearing upon Platyceras, are of unusual significance as furnishing a solution to certain important morphological problems relative to the paleozoic crinoids. From time to time paleontologists have mentioned the occurrence of Platyceras attached to crinoids, and numerous explanations have been advanced, but it was not until 1873* that the probably correct solution was given. A discussion of the various explanations offered prior to this date is not necessary since some of them have been fully considered by Meek and Worthen, who have also pointed out clearly the improbability of the claims of the Austinst in their elaborate and highly imaginary account, and others, that the crinoids perished while in the act of devouring the Platyceras. Not only is this view highly improbable but its absurdity is only too manifest when the character of the food of both these * Geol. Ill., vol. v, p. 334 et seq. + Loc. cit. Recent and Fossil Crinoidea. |