It is important not to overlook the enormous part played by variations in the level of the land relative to that of the ocean in past ages. For instance, there is abundant evidence to show that practically the whole of Great Britain has been repeatedly submerged to great depths and has been raised to heights far greater than its present average. These oft-repeated risings and settlings have caused great changes in the nature of the deposited materials so that in the Coal Measures, for example, there are deposits of obviously fresh-water origin sandwiched in between others undoubtedly marine. It can readily be understood, as stated by Arber (24), that if, at a given period, the dry land during the formation of the Coal Measures gradually subsided, it would first be covered with clear water, whilst from those portions of the area which occupied the higher ground the rivers and streams continued to pour into their estuary a large amount of fresh-water material. Later, a stage would be reached when mud of marine origin invaded the area and covered the previous deposits. When, after an indefinitely long period, the ground again rose, fresh-water deposits might again form, and this alternation of marine and fluviatile deposits appears to have been repeated with great frequency during the Carboniferous period. In the Lower Coal Measures of Yorkshire and Lancashire, Stopes and Watson (23) have shown that the shales forming the roof of the Upper Foot Coal were derived from drifted sediments of marine origin. Precipitated Clays. If the plasticity of some clays is really due to the colloidal nature of their particles, it is obvious that they must have been formed by a process of coagulation or precipitation at a distance from the site of the minerals from which they have been derived. According to the 'colloid theory,' felspar and other alumino-silicates are decomposed by 'weathering,' the chief effect of which is the formation (by hydrolysis) of a colloidal solution of 'clay.' This apparently clear solution flows along in the form of a small streamlet, joins other streamlets and continues its journey. So long as it is quite neutral or contains free alkali the solution will remain practically clear, but as soon as acids enter the stream, or are formed in it by the decomposition of organic matter, a coagulation of the colloidal matter will commence and the amount of 'clay' thus thrown out of solution will depend on the amount of such free acid. If the coagulation or precipitation occurs in still water, the 'clay' will be deposited almost immediately, otherwise it will be carried forward until it reaches a place where it can be deposited in the manner already described.