_The Lias._ The British Lias deposits are divided into the Lower Lias, the Marlstone, and the Upper Lias corresponding in general terms only with the Sinemurian, Liassian, and Toarcian. The Marlstone is separated from the Upper and Lower Lias on account of the greater percentage of carbonate of lime which it contains, so that the bands of argillaceous limestone are much more marked in the Marlstone than in the upper and lower divisions, which consist chiefly of clay. The three divisions possess very much the same characters throughout the country, though the presence of the Mendip ridge and its continuation beneath London is marked by the attenuation of this and succeeding strata, and by the conglomeratic character of some of the Liassic strata where they abut against it. The British Lias, as a whole, seems to have been deposited in a fairly shallow sea at no great distance from the land. It passes down conformably into the Rhætic beds, indeed the zone of Ammonites (_Aegoceras_) _planorbis_, referred by British geologists to the Lower Lias is included by some continental writers with the Rhætic beds, and the plane of demarcation here as in other cases is conventional. _The Lower Oolites._ Of all the British strata, these perhaps cause most trouble to the learner, on account of the different nomenclature applied to the rocks in different parts of England, and the rapid variations in lithological character, when the beds are traced laterally. The following divisions are usually adopted for the beds of the south-western counties where the most marked marine development occurs:-- Cornbrash, Forest Marble, Great Oolite (with Bradford Clay), Fuller's Earth, Inferior Oolite. Of these divisions, the uppermost one, the Cornbrash, though thin, retains its characters with great constancy across the island. Of the others the Forest Marble may be looked upon as a local development of the upper portion of the Great Oolite, and the Fuller's Earth is a local deposit, so that the Inferior Oolite and Great Oolite constitute the important divisions of the Lower Oolites. The variations in the characters of the rocks may be best shown in tabular form.