In examining things which actually exist, and which have proceeded in a certain order, it is natural to look for that which had been first; man desires to know what had been the beginning of those things which now appear. But when, in forming a theory of the earth, a geologist shall indulge his fancy in framing, without evidence, that which had preceded the present order of things, he then either misleads himself, or writes a fable for the amusement of his reader. A theory of the earth, which has for object truth, can have no retrospect to that which had preceded the present order of this world; for, this order alone is what we have to reason upon; and to reason without data is nothing but delusion. A theory, therefore, which is limited to the actual constitution of this earth, cannot be allowed to proceed one step beyond the present order of things. But, having surveyed the order of this living world, and having investigated the progress of this active scene of life, death and circulation, we find ample data on which to found a train of the most conclusive reasoning with regard to a general design. It is thus that there is to be perceived another system of active things for the contemplation of our mind;--things which, though not immediately within our view, are not the less certain in being out of our sight; and things which must necessarily be comprehended in the theory of the earth, if we are to give stability to it as a world sustaining plants and animals. This is a mineral system, by which the decayed constitution of an earth, or fruitful surface of habitable land, may be continually renewed in proportion as it is wasted in the operations of this world. It is in this mineral system that I have occasion to compare the explanations, which I give of certain natural appearances, with the theories or explanations which have been given by others, and which are generally received as the proper theory of those mineral operations. I am, therefore, to examine those different opinions, respecting the means employed by nature for producing particular appearances in the construction of our land, appearances which must be explained in some consistent mineral theory.