when with Grove and Lockyer we try to reach back to an antecedent condition, when there are neither solid masses nor nebulæ, but only an inconceivably tenuous and universally diffused medium made up of an embryonic matter, which has not yet even resolved itself into chemical elements. How this could establish any motion within itself tending to aggregation in masses, is quite inconceivable. To plodding geologists laboriously collecting facts and framing conclusions therefrom, such flights of the mathematical mind seem like the wildest fantasies of dreams. We are glad to turn from them to examine those oldest rocks, which are to us the foundation stones of the earth's crust. [4] "Stellar Evolution." [5] Other facts favour the shorter time (Clarence King, _Am. Jl. of Science_, vol. xlv., 3rd series). What do we know of the oldest and most primitive rocks? At this moment the question may be answered in many and discordant ways; yet the leading elements of the answer may be given very simply. The oldest rock formation known to geologists is the Lower Laurentian, the Fundamental Gneiss, the Lewisian formation of Scotland, the Ottawa gneiss of Canada, the lowest Archæan crystalline rocks. This formation, of enormous thickness, corresponds to what the older geologists called the fundamental granite, a name not to be scouted, for gneiss is only a stratified or laminated granite. Perhaps the main fact in relation to this old rock is that it is a gneiss; that is, a rock at once bedded and crystalline, and having for its dominant ingredient the mineral orthoclase, a compound of silica, alumina and potash, in which are imbedded, as in a paste, grains and crystals of quartz and hornblende. We know very well from its texture and composition that it cannot be a product of mere heat, and being a bedded rock we infer that it was laid down layer by layer in the manner of aqueous deposits. On the other hand, its chemical composition is quite different from that of the muds, sands and gravels usually deposited from water. Their special characters are caused by the fact that they have resulted from the slow decay of rocks like these gneisses, under the operation of carbon dioxide and water, whereby the alkaline matter and the more soluble part of the silica have been washed away, leaving a residue mainly silicious and aluminous.[6] Such more modern rocks tell of dry land subjected to atmospheric decay and ram-wash. If they have any direct relation to the old gneisses, they are their grandchildren, not their parents. On the contrary, the oldest gneisses show no pebbles or sand or limestone--nothing to indicate that there was then any land undergoing atmospheric waste, or shores with sand and gravel. For all that we know to the contrary, these old gneisses may have been deposited in a shoreless sea, holding in solution or suspension merely what it could derive from a submerged crust recently cooled from a