Whichever classification be adopted (and each is useful for special purposes), it must be noted that no hard and fast line can be drawn between one division and another. A rock may be partly arenaceous and partly calcareous, composed of a mixture of sand and lime, and the same rock may similarly be partly mechanically and partly organically formed, the sand being due to mechanical fracture, and the lime to the agency of organisms, and so with the other divisions. As many of the changes which have occurred in past times have been concerned in destruction and obliteration, whilst deposition is the cause of preservation, the study of deposits is peculiarly adapted for testing the truth of the grand principle of geology that the changes which have taken place in past times are generally speaking similar in kind and in intensity of action to those which are in progress at the present day, and a study of the modern deposits is specially important as throwing light upon the characters of those which have been formed in past times. It will be abundantly shown in the sequel that the deposits of the strata are in general comparable in all essential respects with those which are being formed at present, and accordingly they give most valuable indications as to the nature of the physical and other conditions under which they were laid down. The desert sand, the precipitate of the inland sea, the reef-limestone and many another deposit can thus be detected by an examination of their lithological characters, combined with consideration of other kinds of evidence. The petrology of the sedimentary rocks is still in its infancy, though much has already been done, but it offers a wide field of inquiry to the field-geologist and worker with the microscope[8]. [Footnote 8: The student will do well to consult _The Challenger Report_ by Messrs Murray and Renard (1891), for information concerning many modern sediments, and Harker's _Petrology for Students_ Section D, for general information on the Petrology of the Stratified Rocks.] CHAPTER IV. THE LAW OF SUPERPOSITION.