In the first case, the very small proportion of carbonate of lime which is held in solution will necessarily render it a powerful cement, since the density and compactness of a precipitate will generally vary, inversely as the rapidity with which it is deposited. This fact is familiarly illustrated by the obstinate adhesion of calcareous incrustations to the interior surfaces of water decanters. In the second case, wherein a sulphuric salt would appear to act the part of a cement, it may be observed, that the _sulphatization of pyrites in the presence of calcareous matter is a very general source of gypsum_. The granular gypsum from the Falls of Niagara, which is described by Dr. Kidd as being "as white as snow," owes its origin to a natural process of this decomposition; for I am informed by Dr. Maclure of Philadelphia, who has visited the spot, that it is formed in consequence of the action of water upon decomposing slate, which contains numerous veins of _carbonate of lime_ and _sulphate of iron_. I have also in my possession a series of incrustations which were taken out of steam boilers in Cornwall, one of which presents an admirable instance of the formation of _sulphate of lime_, its surface being beautifully studded with its crystals; the water which supplied the boiler, and by the evaporation of which this substance was deposited, was derived from a mine in clay-slate intersected with veins of _Pyrites_ and _carbonate of lime_." "With regard to the third species of cementing matter, viz. _Oxide of Iron_, it is scarcely necessary to state, that in the induration of mineral bodies Iron has been long known to act a very important part; the most superficial observer must have noticed the concretions which so frequently appear on the beach around a rusty nail, or any fragment of iron, while the mineralogist must be acquainted with the proofs which Mr. Kirwan has collected in support of the fact. Nor is the part which it performs in the disintegration of mineral bodies less obvious; by its agency we have seen a loose sand become a hard rock, but if we extend our inquiry we shall find that _Iron_ by attracting a farther proportion of _oxygen_ from air or moisture, soon crumbles into dust, and thus proves the immediate cause of the decomposition of that very rock, of which it formerly constituted the indurating ingredient. In this, as in every other operation, Nature preserves her uniformity, producing the most diversified and opposite effects by the modified application of the same principles." For this long digression we feel conscious that some apology is necessary; the extreme interest as well as novelty of the phenomenon will at once suggest a sufficient excuse to the geologist; and to other observers it may at least be pleaded in extenuation, that they have lost nothing by the delay, for it has been in a district which offers but few objects of amusement or instruction.