The distribution of land mollusca still remains one of the most perplexing of the problems to be solved by the geologist. Sir Charles Lyell seems to have been especially strack with their capricious distribution, particularly in Madeira. The facts are still substantially as stated in the tenth edition of his “Principles of Geology,” for Mr. Leacock, in showing me the results of many years collecting in Madeira, observed that his researches had not modified them in any important particulars. Nearly all the species are peculiar to the Madeira Archipelago, and the remarkable fact about their distribution is that, though there are 56 species in Madeira proper, and 42 in Porto Santo, only 12 are common to both islands, though in sight of each other. Still more unaccountable it seems, that of 19 species found on the Dezertas, three barren rocks which appear but little detached from the main island, only 12 are common to Madeira, and even each of these islets has species and varieties peculiar to itself. But two species of land-shells are in fact common to Madeira, the Dezertas and Porto Santo. Sir Charles Lyell infers the great antiquity of the Archipelago from this, contrasting it with the far more extensive group of the British Isles, numbering 200 islands, not one of which have yet developed peculiar species. The fact that such narrow seas have sufficed to keep the land-mollusca distinct conclusively proves, in his opinion, that they have no ready means of dispersal, and that their passage across even the narrowest sea must be of such extraordinary rarity that the possibility need hardly be taken into account. The presence of the same species throughout the whole of the British Islands is thus the strongest argument in favour of their having been joined together and to Europe at a very recent date.

Two species, or at least two well-marked varieties of one species, occur in the lower beds of the Inferior Oolite of Yorkshire, which cannot be referred to Actœonina, but which are provisionally placed under Actœon. The type is the well-known Auricula Sedgvici, Phil., characteristic of the Dogger, though far from common, and almost impossible to obtain in a perfectly satisfactory state. Hence the chief difficulty.

In a short series of papers in this Magazine on How to work in the Archœan Rocks, I briefly set forth the principles of correlation which seemed to me available, and pointed out the precautions which should attend the application of each test. I discussed the evidence from (1) Organic remains; (2) Order of superposition; (3) Included fragments; and (4) Mineral composition. The lastnamed criterion has the widest application, and therefore it was necessary to define its scope with great care. I showed that its value largely depended upon accessory considerations. I insisted that rock-groups should be compared as a whole, that the degree of metamorphism was an important factor in the evidence, that a valuable auxiliary test was to be found in similarity of succession, that the proof of a Pre-Cambrian age enormously reduced the chances of error, and that, in some cases, the origin of the deposits was a useful aid in correlation. I recall attention to these points, because, at a recent debate in the Geological Society, some of my critics seemed to believe that I had been attempting to determine theage of formations by the comparison of a few hand-specimens. They omitted to take fully into account the subsidiary evidence which has been accumulated during the last decade. But even if we entirely ignore this mass of proof, it is surely of interest to point outlithological resemblances between rock-systems, even if we are not prepared to correlatethem. Twilight is better than absolute darkness. We may say that the Pebidian of Hicks is like the Lower Taconic of Emmons, without asserting the contemporaneity of the two groups; and then leave the lithological hint at the mercy of the evolutionary laws which will either destroy it or develop it into a theory.

By the kind intervention of Sir William Dawson, the Rev. H. H. Higgins has sent me two new Carboniferous Insects of sufficient interest to bring to the attention of English palæontologists. They are both preserved in the Liverpool Museum, one having been presented to it by Major Chambers as long ago as January, 1858; while the other was mentioned by Mr. Higgins in his presidential address to the Liverpool Naturalists' Field Club in 1871, figured on the plate of the Ravenhead fossils in the Liverpool Museum (fig. 15) which accompanies it. Both the wings are gigantic.

Since a “shear” is mathematically the same as a compression in one direction with a compensating expansion in a direction atright angles to it, the consideration whether slaty cleavage can be ascribed to movements of this character resolves itself into the question whether the cleaved rocks have or have not suffered a total diminution of bulk. The most convenient way of treating thequestion is by discussing, as I did in my former paper (p. 15), the form of the “ellipsoid of distortion.” The ellipsoid produced by a pure shear would be one of three unequal axes, of which the second or mean axis would be a geometric mean between the other two: whereas, if the expansion did not compensate the compression, the second axis would be greater than this geometric mean, and if the expansion were slight, the second and greatest axes would be nearly equal. The facts seem to accord with the latter supposition, and the diminution of bulk thus indicated agrees with what might be a priori expected.