+-----------------------------+---------+--------+ | | VII. | VIII. | +-----------------------------+---------+--------+ | SiO_{2} | 45·92 | 45·38 | | Al_{2} O_{3} | 18·16 | 16·62 | | Fe_{2} O_{3} | 1·18 | 4·06 | | FeO | 9·27 | 8·63 | | MnO | 0·19 | 0·14 | | CaO | 7·19 | 8·19 | | MgO | 10·07 | 9·41 | | K_{2}O | 1·78 | 0·71 | | Na_{2}O | 2·12 | 2·20 | | H_{2}O and Loss on Ignition.| 4·22 | 4·34 | | Insoluble Residue. | 0·04 | 0·08 | | Total. | 100·14 | 99·76 | | Specific Gravity. | 2·96 | 2.99 | +-----------------------------+---------+--------+ The two rocks here analyzed, likewise that from the crag south of Castell and that from the cliffs at the southern end of the promontory between Ramsey Sound and Pen-y-foel, show under the microscope a general similarity of composition and structure. They present a variable quantity of a base, which under a ⅕ objective is resolved into ill-defined coalescent globulites and fibre-like bodies, that remain dark when rotated between crossed nicols. In some varieties, as in part of Rhosson Crag, the base is nearly lost in the crowd of crystalline constituents; in others, as in the crag south of Castell, it forms a large part of the whole mass, and may be seen in distinct spaces free from any crystalline particles. Through this base are diffused, in vast numbers, irregularly-shaped grains of augite, seldom showing idiomorphic forms. These grains, or granules, may perhaps average about 0·003 inch in diameter. Plagioclase is generally hardly to be recognized, though here and there a crystal with characteristic twinning may be detected in the base. Magnetite occurs abundantly--its minute octahedra, with their peculiar colour and lustre, being apparent with reflected light on the fresher specimens, though apt to be lost as diffused ferruginous blotches in the more decomposed varieties. But perhaps the most remarkable ingredient is olivine. Red hæmatitic crystals are visible, even to the naked eye, dispersed through the groundmass of the rocks. With a lens these may be observed to be orthorhombic in form, and to be evidently pseudomorphs after some mineral which has been converted chiefly into hæmatite. Such red pseudomorphs are common in Carboniferous and Old Red Sandstone lavas, where in some cases they appear to be after hornblende, and in others after augite, but occasionally are suggestive of olivine, though with no trace of the original substance of that mineral. In the lava associated with the tuffs at the south end of the promontory between Ramsey Sound and Pen-y-foel, however, certain large, well-developed pseudomorphs are undoubtedly after olivine.