Of the various rocks of which the Earth's surface is composed, there are some which present very satisfactory indications of their origin; while others, when considered in that respect, seem to be placed almost beyond the reach of hypothesis. The Granite and Trap rocks, by extending their ramifications among their contiguous neighbours, distinctly avow, not only a subsequent formation, but, in many instances, they present the most unequivocal marks of an igneous origin. The Limestones and Sandstones, however, are equally constant in their indications of aquatic deposition, although, at the same time, the assistance of another agency may sometimes be inferred, when their consolidation and position are taken into the question. Of all the rocks with which I am acquainted, there is none whose formation seems to tax the ingenuity of theorists so severely, as the White Limestone or Chalk, in whatever respect we may think fit to consider it.