at once struck with the very different appearance of these compared with those just described. Take, for instance, a thin slice of a sandstone, no matter how fine-grained it may be; it is seen to be an aggregate of particles, more or less water-worn, of quartz and other minerals, evidently derived from the breaking up of older rocks. Clays, shales, and most of the slate rocks present a somewhat similar appearance, although with a more minute structure, which in the slatey rocks is sometimes considerably affected by the effects of pressure, as well as by other agencies since their first deposition. The microscope, again, will often bring to view in these rocks very numerous remains of organisms. Some of the clays and shales and limestones contain foraminifera, diatomaceæ, and other fossil traces of the life of the period. Occasionally rocks are found, such as some of the foreign slates and clays, that are composed of little else than diatomacea and sponge spicules. The tripoli and semi-opal of Bilin, in Bohemia, present fine examples of such rocks. Chalk, too, when subjected to microscopic examination, reveals instantly its purely organic origin; it is seen to be built up almost entirely of minute organic bodies-foraminifera, sponge spicules, fragments of bryozoa, &c. So bog iron ore can be traced to a similar origin. The older limestones, also, even when no fossils appear to be present, are seen, when sufficiently thin sections are prepared, to be similarly composed; and the microscope thus helps us to correlate the limestone and the chalk of former ages with formations of a like nature, accumulating at the present day in the bed of the ocean. To the microscope we owe the discovery of what is possibly the earliest existing trace of organic life on our earth; the eozoon canadense, which is, if the opinions of those most competent to judge be accepted, the remains of a gigantic foraminifer, entombed in the laurentian serpentine. It is, perhaps, hardly necessary to observe how the microscope will also enable the skilled observer to determine from a small fragment of bone or of a tooth the nature of the animal to which it belonged; or, once more, in the faint traces of vegetable origin frequently met with in certain rocks, to gather some idea of the plant life of the period. We all know how abundantly a microscopic examination of coal and of the carboniferous vegetation has repaid the labour spent upon it, enabling us to form tolerably correct notions as to the nature of the vast flora which clothed a great part of the earth's surface in bygone ages. With these scanty and, I fear, very imperfect notes upon a wide and most interesting field of study, I bring my paper to a close, and I trust that it will not be very long before we find many English geologists awaking to the great importance of this branch of their science. That it will well repay the most earnest attention there can be little doubt. To the petrologist, the mineralogist, and the physicist the intelligent use of the microscope is invaluable, and I need scarcely add, that in this, as in every branch of scientific inquiry, every fresh glimpse we obtain brings new beauties to light. EXPLANATION OF PLATES. CXVI. FIG. 1. Quartz, Quartziferous-Porphyry, Saulieu, polarised. FIG. 3. Triclinic Felspar, polarised. FIG. 4. Chlorite, Chloritic Schist, nat. FIG. 5. Biotite, Germany, seen with polariser alone. FIG. 8. Olivine, Somma, Vesuvius, &c., polarised. FIG. 9. Leucite, Somma, Vesuvius, polarised. FIG. 10. Nepheline and Apatite, Katzenbuckel, nat. FIG. 11. Nosean, nat. FIG. 12. Hæmatite and Magnetite, nat. (All magnified 26 diam.) CXVII. FIG. 1. Gas Cavities, in Lava, Hecla. FIG. 2. Water Cavities, in Quartz, Cornwall. FIG. 3. Glass Cavities, in Quartz, Cornwall. FIG. 4. Glass and Stone Cavities, in Lava, Hawaii. FIG. 5. Calcite, Marble, Ephesus. FIG. 6. Aragonite, Marble, Carrara. FIG. 7. Chalcedony, Cornwall. FIG. 8. Fibrous Zeolite, in Dolerite, Germany. FIG. 9. Mica, in Trachyte, Germany. FIG. 10. Organisms, in Limestone, Derbyshire. (Magnified 56 diam.) 20 THE NEBULAR HYPOTHESIS: ITS PRESENT CONDITION. BY JOHN J. PLUMMER, M.A. IN seintion, so the whole range of science there is no theory which has attracted so much attention, has passed through so many vicissitudes, and has been so earnestly and fondly supported in the face of opposing evidence, as the nebular hypothesis of Laplace. This has arisen, doubtless, to some extent from the respect due to the very eminent astronomers by whom it was first suggested and promulgated, but perhaps still more to the nature of the hypothesis itself, and to the fact that many otherwise unexplained phenomena find in it a satisfactory solution. The whole course of scientific progress has led us to look for the most simple laws in order to explain the most apparently complicated results; and such a law the nebular hypothesis would become, were it possible to give to it such a high degree of probability as at present serves for the demonstration of the Newtonian law of gravitation, or of the undulatory theory of light. That it may one day attain to this degree of certainty, and be recognised as an established truth, is the hope of many who are fascinated alike by its simplicity and its comprehensiveness- --a hope that has often served to sustain it when the bulk of evidence has not appeared to be in its favour, and one in which the writer to some extent indulges, although well aware that it may require to undergo considerable modification before it reaches that exalted position. Previously to the revelations of the spectroscope, the nebular hypothesis stood at a very low ebb. The gradually increased powers of telescopes, culminating in the gigantic reflector of Lord Rosse, had one by one reduced the number of the so-called nebulæ, by resolving them into clusters of distant stars, very closely packed together, until, although a large number still continued nebulous in appearance under all circumstances, it seemed very probable indeed that indefinitely increased telescopic power was all that was needed to resolve the remainder. Still it retained a few believers, loth to relinquish the insight |