fluid-inclusions, until another bundle of fibrolite is reached on the further side of the grain, and the rock-fissure re-appears as a minute dark crevice cutting across the needles as before. All these occurrences are illustrated in the drawing made under a 4-10 inch objective, by a Camera Lucida, presented in Plate X. The quartz-field in the center, shot through with but a few blades of fibrolite, is traversed by several anastomosing fissures, as lines of inclusions, which at one end are cut off by a composite scale of muscovite and opaque biotite, and at the other are transformed into the veins of fibrolite-material. The occurrence of the fluid-inclusions in planes, which in most cases lie obliquely to that of the thin section, is easily studied by deeper focussing, but could not of course be represented in the drawing; to this fact is due the occasional appearance in the latter of superposition of the inclusions upon a fibrolite needle, or the slight projection of a needle across the fissure within the quartz-field. All these phenomena are interesting evidences of the microscopic results of the internal and gradual movements within the mother-rock in the course of folding. The mass has been repeatedly seamed by minute fissures, yielding the plasticity long-recognized in rock-masses of apparently the greatest rigidity, and repeatedly re-cemented by siliceous films deposited out of the concentrated and heated solutions which saturated the rock. The process was closely akin to that of the repeated fissuring and re-cementation in the ice-grains of a glacier. The huge veins, selvage, and slicken sides which are the familiar macroscopic results attendant upon such movements, here show in some respects their microscopic counterparts. By the slow grinding action, the greatest width of the fissure was attained and a fragmentary vein produced, where the softer material (fibrolite) occurred along its course: some needles being simply torn out or pulled apart, but not broken, where the rending force diminished, near the thinning out of the fissure. By the fortunate co-incidence of such fissures of sufficient length, passing across both quartz and a fibrous mineral like fibrolite, with their subsequent partial occupation by dissolved silica (a common cementing material in many rocks), the rock under investigation apparently presents clear evidence that, in this case at least, the extension of the fluid-inclusions in planes indicates for them a fissure-origin, secondary to, and it may be far later than, the genesis of the rock. It is also easy to understand how readily, as such a capillary fissure is gradually filled up, portions of its area may be entirely occupied by gas, steam, or a liquid, and resist and prevent the deposition of silica, until a barrier had been formed. The constant tendency to crystallization within this siliceous deposit produced the abundant cavities of crystalline form, frequently modified by partial re-solution and erosion of angles. The accidental enclosure in each case of gas or vapor or liquid under heavy pressure, with an additional introduction or escape of any one of these before the barrier became complete and the cavity hermetically sealed, and with the subsequent contraction on cooling, afforded all the conditions apparently necessary for the plane arrangement of the inclusions as now found. Thus, too, they occur completely or partially filled with either gas or liquid, but with a general correspondence in regard to contents, in contiguous groups of inclusions. It may be further stated that just such parallel lines of fluid inclusions cross the quartz-grains in many other gneisses and granites of this country. From these facts we may conclude that in many rocks those groups of fluid-inclusions, which are marked by a plane arrangement, are of secondary origin, resulting from a late injection, as suggested by Vogelsang-not exactly of cleavage cracks, however, but of rock fissures. In such cases they can throw light only on the conditions of metamorphism in a rock, rather than on those of its origin. Doubtless in the same rock, or even in the same quartz-grain, fluid-inclusions of both kinds may co-exist, and a careful distinction between them, whenever possible, is requisite in investigations like those of Sorby already alluded to. The great variations in relative volumes of bubble and liquid and fluid contents in general, in closely contiguous cavities, often remarked by many observers, may be in part due to this difference in their origin, and may yet serve as a means of distinction in certain cases; e. g. where fissure or secondary inclusions may be distinguished from the primary by the co-incidence of certain characteristics (the same contents, or the same volume-relationship of bubble and liquid) along cer tain otherwise invisible planes. Evidently, that careful observer, Sorby, was on his guard in this respect, as he makes the following statement concerning the fluid cavities in the quartz of quartz-veins: "Sometimes we may distinctly see that the quartz has been cracked, and the cracks afterwards filled up with quartz. This appears in some cases to have taken place at a low temperature and explains why bands of cavities occasionally occur with vacuities relatively less than those in the fluid cavities of the general mass." I believe, however, that in some instances, as in this very rock, fissure-inclusions of this class are quite difficult to identify, from either their sparseness or their great number and irregularity. It may be further suggested that since such inclusions appear to have been formed during the folding or extrusion of a mass, they present the exact conditions for the excess of pressure indicated in the calculations of Sorby and Ward, and by them attributed to the same cause Again, the distinctions between rocks founded upon the presence or abundance of fluid-inclusions in certain groups, such as that proposed by Zirkel* between the metamorphic and younger eruptive granites of the West, on account of the frequent poverty of the former in liquid-inclusions, may, although local, yet meet with striking exceptions and need careful reconsideration. In the case of this metamorphic gneiss of Westchester and New York counties, which is certainly a member of the Montalban group, such a scheme, if pressed to a conclusion, would conflict with the idea of Archean age, and rather tend to corroborate the current suspicion, on other grounds, of its Silurian origin. Finally, it may be briefly stated that the same fissure-inclusions in planes have been commonly observed in the quartzgrains of American sandstones, terminating abruptly at the outlines of the grains. They occur also in the quartz-grains of many metamorphic or conglomeritic gneisses of Wisconsin, North Carolina, Colorado, etc., and with the same abrupt terminations at the limits of the grains, which, as Zirkel observes of the last locality, "makes them appear like worn elastic fragments." *Zirkel, loc. cit., 55. +Expl. of 40th Par. VI., 36, 58. |