In passing on to the consideration of the rate at which the various strata were formed, it will be well, I think, to group the rocks under six heads. First, Mechanical deposits formed in deltas, lakes, and estuaries; secondly, Mechanical deposits along the shoreline, in large bays, or constricted seas, such as the German Ocean; thirdly, Chalk; fourthly, Coral Limestone; fifthly, Coal; and lastly. Volcanic Rocks.