The task of the stratigraphical geologist is really to learn the evolution of the earth, in its changes from the simple to the more complex conditions, and it is quite obvious that it is unnatural to attempt any study of evolution by working backward. For this reason the study of the sediments will be here made in the order which is usually adopted, by passing from the older to the newer, and from the simple to the more complex. The British strata will be mainly considered, though references will frequently be made to their foreign equivalents, and a fuller account of the latter will be added when the British strata are abnormal, as are those of Triassic times, and also when a period is not represented amongst the strata of the British Isles, as for instance, the Permo-Carboniferous and Miocene periods. The student is recommended to refer constantly to good geological maps of the British Isles, of Europe, and of the world. Of maps of the British Isles, mention may be made of Sir A. Ramsay's geological map of England, Sir A. Geikie's map of Scotland, and his map of the British Isles, J. G. Goodchild's map of England and Wales, a map of Europe by W. Topley and one of the world reduced from that by J. Marcou, accompanying the first and second volumes of the late Sir J. Prestwich's _Geology_. For special purposes more detailed maps will be studied, including the one-inch maps of H. M. Geological Survey, and the index map on a smaller scale. Lastly, for an account of British Geology, reference must be made to H. B. Woodward's _Geology of England and Wales_, where the British formations are described in order, and to W. J. Harrison's _Geology of the Counties of England and Wales_, where the stratigraphical geology of the country is given under the head of the different counties, which are taken in alphabetical order. In concluding this chapter, it is hardly necessary to say that every opportunity of studying the characters of the deposits and their fossils in the field should be eagerly seized, and that much information may be acquired even on a railway journey, especially as to the influence which the deposits exert upon the scenery of a region[47]. [Footnote 47: In the first edition of H. B. Woodward's _Geology of England and Wales_, an account of the geology of the main lines of English railways is given, which is omitted in the later edition. It is well worth consulting by those who take a long journey, and it will be found useful to take a geological map with one on the journey so as to discover when one is passing from one formation to another.] CHAPTER XII. THE PRECAMBRIAN ROCKS.