but above the Keuper beds we find a group of deposits of some importance, which shew affinities with both Triassic and Jurassic rocks, which may be looked upon as true passage beds, though they are generally placed in the Triassic System. They are known as Rhætic or locally in Britain as Penarth Beds. The Muschelkalk is usually considered to be unrepresented in Britain, and accordingly the British deposits may be, and are usually grouped as under:-- Rhætic or Penarth beds Keuper { Keuper Marls { Keuper Sandstones [Muschelkalk] absent { Upper Red and Mottled Sandstones Bunter { Bunter Pebble Beds { Lower Red and Mottled Sandstones. The threefold grouping has been applied more or less universally, but when used outside the north-west European area, it loses its significance, as the conditions which enable one to differentiate the rocks of the three divisions were naturally only prevalent over a limited area. _Description of the strata._ The British Triassic rocks possess a certain sameness as regards their general characters, consisting mainly of mechanical sediments coloured red by peroxide of iron, with occasional chemical precipitates of rock-salt and gypsum. They have a wider distribution over Britain than have the Permian rocks, and the lithological characters of the different subdivisions do not as a rule vary to a remarkable degree when traced laterally. The differences in detail in the characters of the various deposits are noteworthy, and an explanation of the exact origin of some of these abnormal deposits which will satisfy everyone is not yet forthcoming. Leaving the details out of consideration for the moment, and looking at the general aspect of the deposits, the prevalence of conditions generally similar to those which existed over the British Isles in the preceding Permian period is decidedly indicated by the nature of the strata, though the continental conditions appear to have been more widely established over our area, as shewn by the general absence of any calcareous deposits resembling the Magnesian Limestone. We find chemical precipitates, millet-seed sandstones, and scree-like breccias in the British Triassic rocks as well as in those of Permian age, and the paucity of a marine invertebrate fauna in the Triassic rocks of Britain is even more apparent than in the Permian strata. It is only at the extreme close of the Triassic period, during the deposition of the rocks which are admitted on all hands to be of Rhætic age, that we note the incoming of those marine conditions over our area, which prevailed so extensively, with few local exceptions, during the remainder of the Mesozoic and the early part of Tertiary times; the Rhætic beds, in fact, mark the commencement of the third marine period. Referring to the strata in further detail, we may proceed to