Denudation is the laying bare of one stratum, or portion of a stratum, by the removal of another. It is carried on principally by rains and rivers and the action of the sea-waves upon the sea-border. To the last-mentioned agency the geologist is highly indebted; to the others also he owes a debt: but consider how they all do their work. Much of the material dealt with they pound into mud or sand, and in these any fragments that escape the trituration are, sooner or later, again buried. They may tear open the rocks, and expose for a brief period the most interesting and unique fossils; but, unfortunately, they carry on their work by night as well as day, on desolate coasts, in places where the Palæontographical Society has no missionaries, or when the missionary, if there be one, is in-doors writing a book; so that a very small percentage of all that might be discovered is ever actually found. In the artificial denudation of mining and quarrying, though the rude forces of Nature are dispensed with, the enlightened hammer of the geologist can do very little by itself. In most cases it can but follow where commercial enterprise leads the way, and be grateful for permission to rummage among the _débris_, when pickaxe and blasting have done their work. The chances against a fossil’s being found to any useful purpose in quarrying are very numerous. The rock must chance to split so as to disclose it; the workman must chance to notice it; he must chance to have knowledge enough to think it worth notice; have time enough to stop from his work and take it; have sense enough to keep it safe; have memory enough to recollect where he hides it; and, lastly, have the luck to meet with a customer who knows its scientific value. Numbers of rare specimens must continually be consigned to the furnace and the limekiln, or buried under mounds and hills of refuse. Sometimes the character of the matrix, by its hardness or its softness, makes it impossible to disengage the fossil without complete disfigurement; sometimes the fossil itself is so fragmentary as rather to confuse than to teach. Dr. Hooker gives an instance, in which a geologist assigned three pieces of fossil-leaf to plants of three different genera, which a subsequent observer maintained to be merely the separated portions of a single leaf of one and the same plant[62].