However this may be, the distribution and extent of these pre-Laurentian lands is, and probably ever must be, unknown to us; for it was only after the Laurentian rocks had been deposited, and after the shrinkage and deformation of the earth's crust in subsequent times had bent and contorted them, that the foundations of the continents were laid. The rude sketch map of America given in Fig. 3 will show this, and will also show that the old Laurentian mountains mark out the future form of the American continent. [Illustration: Fig. 3.--The Laurentian Nucleus of the American Continent, after Dana.] Some subsequent writers have, it is true, treated with disbelief Logan's great discoveries; but no competent geologist who has traced the regularly bedded limestones and other rocks of his original fields of investigation could continue to doubt. On this subject I may quote from my friend Dr. Bonney, one of the most judicious of the builders who undertake hypothetically to lay the foundation stones of the earth's crust for our enlightenment in these later days. In an address delivered at the Bath meeting of the British Association he says:-- "The first deposits on the solidified crust of the earth would obviously be igneous. As water condensed from the atmosphere on the cooling surface, aqueous waste or condensation would begin, and stratified deposits in the ocean would become possible in addition to detrital volcanic material. But at that time the crust itself, and even later stratified deposits would often be kept for a considerable period at a high temperature. Thus, not only rocks of igneous origin (including volcanic ashes) would predominate in the lowest foundation stones, but also secondary changes would occur more readily, and even the sediments or precipitates might be greatly modified. As time went on, true sediments would predominate over volcanic materials, and these would be less and less affected by chemical changes, and would more and more retain their original character. Thus we should expect that as we retraced the earth's course through 'the corridor of time' we should arrive at rocks which, though crystalline in structure, were evidently in great part sedimentary in origin, and should behind them find rocks of more coarsely crystalline texture and more dubious character, which, however, probably were in part of a like origin, and should at last reach coarsely crystalline rocks, in which, while occasional sediments would be possible, the majority were originally igneous, though modified at a very early period of their history. This corresponds with what we find in nature, when we apply, cautiously and tentatively, the principles of interpretation which guide us in stratigraphical geology."[49] [49] "The Foundation Stones of the Earth's Crust," 1888. The extract is slightly condensed.