BY R. H. CAM B AGE. 379 



derived their colouring from the oxidation of magnetite contained 

 in fine volcanic tuff interstratified with the shales. They extend 

 approximately from about Maitland in the north to the westward 

 of Kiama in the south, and near Lithgow in the west, though 

 their north-western extent is not at present definitely defined; 

 and they are overlaid throughout by a considerable thickness of 

 Hawkesbury Sandstone, which in the Parramatta to Penrith, 

 Picton and surrounding districts is again covered by the Wiana- 

 matta Shales. Towards the edges of the above first-mentioned 

 area the chocolate shales thin out, and in some cases have been 

 so denuded as well, that they only appear as reddish-brown bands 

 in the face of the cliflts, particularly on the Blue Mountains or on 

 the talus slopes of ridges capped by Hawkesbury Sandstone; and 

 while having little or no influence on the surrounding vegetation, 

 they afford a splendid geological horizon for the correlation of 

 adjacent rocks. Owing to subsidence of the central part of the 

 coal basin west of Sydney, the Narrabeen Series are found to 

 have dipped from above sea-level at Narrabeen to nearly 900 feet 

 below sea-level on the northern shore of Port Jackson, as proved 

 by the first Cremorne bore. They rise again, however, on the 

 southern side of the basin, and reappear on the coast to the 

 north-east of Otford, nearly 30 miles southerly from the entrance 

 to Port Jackson. That they rise again to the eastward in the 

 Sydne}^ district is shown by the fact that at The Sydney Harbour 

 Collieries' shaft at Balmain the Chocolate Shales are about 950 

 feet below sea-level, while at the first Cremorne bore, which is 

 situated about three miles to the eastward, they are met with at 

 about 890 feet, though the strata are rising more rapidly to the 

 eastward in this latter locality, as proved by the results of the 

 first and second Cremorne bores. At these two before-mentioned 

 extreme points, viz., Narrabeen to Newport on the north, and 

 near Otford on the south, these shales are exposed to a greater 

 extent than is commonly found in any other part of their area, 

 the Hawkesbury Sandstone having been denuded; and it was 

 owing to their exposure and the facilities thus afforded for their 

 study in the northern area that the local name of Narrabeen 



