forest trees, which went to constitute the masses of graphite. Geologists will agree that this is no small assumption to make, since, if true, it may show that there was an abundance of vegetation at a time when animal life was hidden in one or more very obscure forms, one only of which has so far been detected, and whose very identity is strongly doubted by nearly all competent judges. At the same time there _may_ have been an abundance of both animal and vegetable life at the time. We must not forget that it is a well-ascertained fact that in later ages, the minute seed-spores of forest trees were in such abundance as to form important seams of coal in the true carboniferous era, the trees which gave birth to them being now classed amongst the humble _cryptogams_, the ferns, and club-mosses, &c. The graphite of Laurentian age may not improbably have been caused by deposits of minute portions of similar lowly specimens of vegetable life, and if the _eozoön_ the "dawn-animalcule," does represent the animal life of the time, life whose types were too minute to leave undoubted traces of their existence, both animal life and vegetable life may be looked upon as existing side by side in extremely humble forms, neither as yet having taken an undoubted step forward in advance of the other in respect to complexity of organism. [Illustration: FIG 30.--_Lepidodendron_. Portion of Sandstone stem after removal of bark of a giant club-moss] There is but one more form of carbon with which we have to deal in running through the series. We have seen that coal is not the _summum bonum_ of the series. Other transformations take place after the stage of coal is reached, which, by the continued disentanglement of gases, finally bring about the plumbago stage. What the action is which transforms plumbago or some other form of carbon into the condition of a diamond cannot be stated. Diamond is the purest form of carbon found in nature. It is a beautiful object, alike from the results of its powers of refraction, as also from the form into which its carbon has been crystallised. How Nature, in her wonderful laboratory, has precipitated the diamond, with its wonderful powers of spectrum analysis, we cannot say with certainty. Certain chemists have, at a great expense, produced crystals which, in every respect, stand the tests of true diamonds; but the process of their production at a great expense has in no way diminished the value of the natural product. The process by which artificial diamonds have been produced is so interesting, and the subject may prove to be of so great importance, that a few remarks upon the process may not be unacceptable.