19 



passing thi'ough the " Gap," a break in the precipitous and rugged 

 escarpment of Hawkesbury Sandstone, which here unconformably 

 overlies the untilted Silurian rocks. The latter can be seen 

 almost vertical along the river-bed road previously mentioned, 

 with a general north and south strike, the junction of the Silurian 

 and Hawkesbury rocks taking place a little below the escarpment. 



The plateau now reached, and of which Mounts Bulee and 

 Sassafras only form portions, consists of a high tableland of 

 Hawkesbury Sandstone, and possesses many features in common 

 with the physical aspect of the district north of Port Jackson. 

 The Bulee escai'pment exhibits some grand examples of rock 

 weathering, the particular form here taken being the castellated, 

 producing large detached buttresses gradually decreasing upwards, 

 and disintegrating in thin layers or laminaj, like so many 

 pancakes piled one on the other. 



From above Bulee Gap the road proceeds over the tableland, 

 now rising, now descending, but never to any great extent, until 

 the summit of Mt. Sassafras is insensibly reached. A short 

 distance before Mr. Greg's accommodation house comes into view, 

 a marked change takes place in the appearance of the ground, 

 which suddenly passes from the glistening sandy soil of the Hawkes- 

 bury Sandstone to a rich red and brown loam, resulting from the 

 decomposition of the basaltic rock of which the summit of the mount 

 is composed, but when first struck the latter rock appears only as 

 cappings to the low spurs of sandstone. The basalt is described 

 by Mr. C. S. Wilkinson as intrusive,* and I have been favoured 

 by my colleague, Mr. W. Anderson, of the Geological Survey, 

 with the following notes on^ this rock from specimens collected by 

 us. " It is a dolerite consisting of tricliiuc felspar, augite, olivine, 

 and magnetite, but none of the minerals showing much decom- 

 position. The olivine contains many included crystals of 

 magnetite, and neither it nor the augite show distinctly crystalline 

 outlines, the latter occurring as large irregular sliaped pieces, in 

 which are embedded both felspar and olivine, showing that it 

 was one of the latest minerals to crystallize out," 



At the Gap our attention was called by Mr. C. H. Roberts to 

 a large branch of fossil wood, which had been obtained in situ in 

 the Hawkesbury Sandstone. It is silicified, microscopic sections 

 failing to yield more definite details than the feet that the 

 wood is probably coniferous. 



Mt. Sassafras is the highest point on the watershed between 

 the heads of the Clyde River on the south, and the Ettrema and 

 Danjera CreelvS, Ijranches of the Yalwal Creek, a tributary of 

 the Shoalhaven River, on the north. It is stated to be 3,125 

 feet above sea level, and rises considerably higher than the 

 tableland around it. 



* Ann. Report Dept. Mines^ N. S. Wales, for 1885 (1886) p. 132. 



