The county of Cumberland, in which the city of Sydney is situated, and parts of the adjacent counties, are composed almost entirely of a palæozoic formation of great thickness and extent. The principal materials of this formation are certain shales and sandstones, with a few associated beds of coal. In the close of the year 1845 I made a short excursion across a portion of this district, in company with the Rev. W. B. Clarke. We carried with us a mountain barometer, and by taking the means of the two sets of observations made in going and returning, got such an approximate estimate of the heights of the ground and the thickness of the rocks, as to enable us to construct a section with a sufficient approach to accuracy to be relied on for my present purpose. This section runs from Liverpool (a town just at the head of the tidal waters of George's River, which falls into Botany Bay) by a slightly winding line, first S.S.W. for about twenty miles, through Campbelltown to Appin, and then about S.S.E. for about eighteen miles to Wollongong in the Illawarra district. Wollongong is on the coast about forty-five miles to the northward of Sydney. From Paramatta by Liverpool to Campbelltown the country is low, gently undulating, and composed (P1. VII.) almost entirely of (No. 1) black and brown shales, with a few thin interstratified beds of sandstone in their lower portion. From Campbelltown to Appin the country rises into bolder undulations, and