the strata on one side of the hill dip to the north, on the opposite to the south, and in the middle or centre of the hill they are nearly perpendicular. Numerous veins of compact quartz traverse the slate in all directions. A quarry, which has been wrought to considerable extent on the east side of the Lion's Rump, and which is shewn in Plate VI. Fig. 1., but on a larger scale than the proportions of the general elevation, affords a fine view of the structure of the clay-slate, and in one place there is a bed of sandstone in the slate. The sandstone, which is of a yellowish-grey colour, is composed of grains of quartz, with disseminated felspar and scales of mica. Lion's Head. The strata of clay-slate continue to the base of the Lion's Head. Here they are succeeded by strata of compact gneiss, which is composed of grey felspar and quartz, with much dark brown mica in small scales. The gneiss is distinctly stratified, and the strata in some places dip under the next rock, which is granite, in others they dip from it. Numerous transitions are to be observed from the gneiss into the granite, and in the same bed of compact gneiss, one part will be gneiss and another granite. Portions of granite of various sizes are imbedded in the gneiss, and numerous blocks of gneiss, varying much in size, are imbedded in the granite. Sometimes the imbedded portions of granite and gneiss are distinctly separated from the surrounding rock; in other instances they are much intermixed at their line of junction, and veins shoot from the imbedded portions of granite into the surrounding gneiss. Beds of granite appear in some places to alternate with the compact gneiss. Veins of granite, varying from a few inches to several feet in width, traverse the gneiss, and are to be observed shooting from the granite, or are contained in the gneiss, and do not appear to have any connection with beds or masses of granite. Granite forms a considerable portion of the Lion's Head. It is a compound of pale red felspar, grey quartz, and brownish-black mica. It is more frequently coarse granular than fine granular, and is often porphyritic. It is occasionally traversed by veins of quartz, or of felspar, or of granite. It does not appear to be distinctly stratified in any part of the mountain. In some parts the granite is intersected by veins of greenstone, and one of these veins (re 2 In short, the mountains and hills of the peninsula of the Cape of Good Hope, are to be considered as variously aggregated compounds of quartz, felspar, and mica, and the whole as the result of one nearly simultaneous process of crystallisation. This view affords a plausible explanation of all the varieties of aggregation, structure, position, and transitions, observed in the rocks of this part of Africa. ART. XI.-Historical Account of the Discoveries respecting the Double Refraction and Polarisation of Light. Communicated by the Author. THE subject of the Double Refraction and Polarisation of Light, though one of the most important and interesting branches of human knowledge, has scarcely, if at all, attracted the attention of English readers. Our chemists and mineralogists have neglected to avail themselves of the lights which it offers to throw upon their respective sciences: Our popular lecturers on Experimental Philosophy have not been aware of the fine experiments and brilliant exhibitions with which it can so liberally supply them; and the greater number of our learned Professors have not yet found leisure to admit it into their course of physical science. This inattention to an inquiry possessing so many claims upon our notice, can have arisen only from an opinion which we believe has too generally prevailed, that the subject of double refraction and polarisation is not susceptible of popular explanation; and from an erroneous notion, propagated by individuals whose pursuits have been eclipsed in the splendour of a new science, that it consists only of insulated facts and extravagant assumptions. One of the principal objects of the present series of papers is to correct these absurd misapprehensions; and we have no doubt that we shall be able to render the subject intelligible to such of our readers as have but a very slender portion either of physical or mathematical knowledge; and to convince those whose attainments are of a higher order, that almost all the phenomena of double refraction and polarisation, intricate and capricious as they appear to be, have been brought under the dominion of general laws, and can be calculated with as much accuracy as 1 Devil's Peak. The most easterly mountain of the groupe we are describing, named the Devil's Peak, agrees with Table Mountain in the nature and arrangement of the rocks of which it is composed. The lower part of the mountain exhibits strata of clay-slate; these, as we ascend, are succeeded by granite, and the upper parts and summit are of the usual varieties of sandstone *. General Result. Such, then, are the grand geognostical features of the mountains near Cape Town, and of those parts of the peninsula which have been examined. Are we to consider all these rocks as having been formed at different times, or are they of simultaneous formation? The advocates for the Plutonic theory maintain, that the slate was first deposited in horizontal strata at the bottom of the sea,-that these strata were afterwards softened by heat, and raised from their original horizontal to their present vertical position, by the action of fluid granite projected from the interior of the earth,—and that after these two operations were finished, a third took place, namely, that of the deposition of the sandstone over the granite and slate. According to this hypothesis, these rocks have been formed at three different periods, the slate first, next the gra nite, and last of all the sandstone; and two of the formations, viz. the slate and sandstone, are of aqueous origin, while the third or granite is of igneous formation. We consider this explanation as unsatisfactory, and are inclined to view these rocks as of Neptunian and simultaneous formation; because they alternate with, and pass into each other, thus exhibiting the same general geognostical relations as occur in formations composed of sandstone and limestone, or of sandstone and gypsum. The junctions of the granite and gneiss, and of the sandstone and slate, do not present any species of veins, or varieties of intermixtures, or of imbedded portions (fragments of the Huttonians), or convolutions, that do not occur at the junctions of universally admitted Neptunian rocks, such as limestone, claystone, gypsum, and sandstone. In Plate VI. is a plan sent by Dr Adam, shewing the form and relative positions of the mountains near Cape Town. In short, the mountains and hills of the peninsula of the Cape of Good Hope, are to be considered as variously aggregated compounds of quartz, felspar, and mica, and the whole as the result of one nearly simultaneous process of crystallisation. This view affords a plausible explanation of all the varieties of aggregation, structure, position, and transitions, observed in the rocks of this part of Africa. Com ART. XI.-Historical Account of the Discoveries respecting the Double Refraction and Polarisation of Light. municated by the Author. THE HE subject of the Double Refraction and Polarisation of Light, though one of the most important and interesting branches of human knowledge, has scarcely, if at all, attracted the attention of English readers. Our chemists and mineralogists have neglected to avail themselves of the lights which it offers to throw upon their respective sciences: Our popular lecturers on Experimental Philosophy have not been aware of the fine experiments and brilliant exhibitions with which it can so liberally supply them; and the greater number of our learned Professors have not yet found leisure to admit it into their course of physical science. This inattention to an inquiry possessing so many claims upon our notice, can have arisen only from an opinion which we believe has too generally prevailed, that the subject of double refraction and polarisation is not susceptible of popular explanation; and from an erroneous notion, propagated by individuals whose pursuits have been eclipsed in the splendour of a new science, that it consists only of insulated facts and extravagant assumptions. One of the principal objects of the present series of papers is to correct these absurd misapprehensions; and we have no doubt that we shall be able to render the subject intelligible to such of our readers as have but a very slender portion either of physical or mathematical knowledge; and to convince those whose attainments are of a higher order, that almost all the phenomena of double refraction and polarisation, intricate and capricious as they appear to be, have been brought under the dominion of general laws, and can be calculated with as much accuracy as that with which the astronomer can compute the motions and positions of the heavenly bodies. In executing this task, we shall explain the phenomena in the order of their discovery. By this means each article will be complete in itself, and the reader will be less dependent on the popularity and perspicuity of our illustrations. In the adoption, therefore, of this plan, the materials may be arranged under five periods. Period I. Containing the discoveries of Erasmus Bartholinus. III. The investigations, of Newton, Beccaria, Martin, V. The discoveries made after Malus's death, by M. Before entering, however, on the first of these periods, we shall endeavour to explain in general what is meant by Double Refraction and Polarisation; for though the explanation of these and other terms will necessarily be introduced in describing the phenomena to which they have been applied, yet there are many of our readers who will content themselves with the slight knowledge conveyed in a definition, without being at the trouble of any farther investigation. If a beam of light from the sun or any other luminous body is made to pass through a plate of glass or a mass of water contained between two parallel plates of glass, it will have the same appearance and the same properties after transmission as before it. Hence the glass and the water refract singly, and all objects seen through them will appear single. If the same beam of light is made to pass through a parallel plate of calcareous or Iceland spar, or of various other crystallised bodies, it will be divided, at its entrance into the plate, into two separate pencils or beams. The crystals which possess this property are called doubly refracting crystals, and the beam of light is said to be doubly refracted, and hence all objects seen through such crystals will appear double. As one of these pencils or images is refracted according to the ordinary |