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22 extinct at the spot where they first began to exist before they could spread in vast numbers over the opposite hemisphere,—would have first to be settled. Again, if the species are not identical, but only representative, is the fact of that representation always to be taken as proof of strict synchronism? May not species representing our Devonian have inhabited the seas of the opposite hemisphere, while our own seas were filled with the Silurian animals on the one hand, or the Carboniferous on the other? Questions like these have yet to be answered before we can determine whether or no strict synchronism can be deduced from the fact of the fossils in opposite hemispheres being representative of, or even if it ever turn out so, identical with each other. At all events, we must not trust too implicitly to single or isolated facts. We must get the series of formations in each case, and compare them with each other, see whether the changes in the one answer to those in the other, and endeavour to trace out some common starting-point of time, before we shall be able to draw clear geological horizons, establish definite chronological epochs common to the whole earth.

Keeping these cautions in view, I should for the present hold the rocks of Australia now under consideration simply as palæozoic, and only assert that their age was included within that of our Silurian, Devonian, and Carboniferous periods.

In the district now described, and which may be called the Sydney district, including the counties of Cumberland and Northumberland, and parts of the adjacent counties, the palæozoic rocks have a rudely basin-formed arrangement. About Illawarra they dip to the north, the coal-measures, exposed in the escarpment overlooking Wollongong, coming down to the level of the sea at Bulli, plunging underneath it, and not again emerging till we reach the Hunter River, where they may be seen dipping to the S. The Sydney sandstone above them (No. 2) forms all the coast line between those points, being generally horizontal, but