PROCEEDINGS OF SECTION D. 475 



and ordinary weathering, and in this way effect the undermining of 

 the sandstone with which they are interstratified, thus producing deep 

 valleys and gorges among the mountains. Although the eastern rivers 

 are short, and, in view of the height of their sources, consequently 

 rapid, they have not yet succeeded in reducing much of their channels 

 to base level, and the work of lateral corrasion referred to by Mr. E. C. 

 Andrews, B.A., (1) is in many places only beginning. The result is 

 that the coastal area, even where the rock is not sandstone, is full of 

 well-sheltered valleys, subject to a good rainfall, and which are nestling 

 under the steep eastern face of the dividing range. 



On the western watershed the streams are very much longer, have 

 a much more even grade, the lateral corrasion is considerable, deep 

 sheltered valleys are practically absent, the descent from the mountains 

 to the low land is gradual, producing an area referred to as the western 

 slopes, and terminating in a vast tract of level plains. 



It will thus be seen that the physiography of the country has a 

 most important bearing upon the flora. With the eastern aspect in 

 New South Wales a wealth of vegetation is produced, even in valleys 

 among the sedimentary formations ; but where the soil is enriched from 

 the decomposition of volcanic rocks — as in parts of the south and 

 north coast — the vegetation can only be described as luxuriant. Plants 

 which require an eastern aspect in New South Wales are practically 

 altogether absent from the western fall. Instances have been noticed 

 where coastal species have crossed to the western side of the main 

 range, but are then restricted to the eastern side of the hills. Interest- 

 ing observations may be made on hills in somewhat exposed situations, 

 the flora on one side, even with the same geological formation, being 

 quite different from that of the other, the regulating factor in such an 

 instance being aspect. 



The question of aspect, therefore, which is an important phase of 

 climate, has a very great influence on the regulation of plant life, and, 

 as previously stated, is dominated in New South Wales by the position 

 of the main range. 



THE COASTAL AREA. 

 For studying the effects of geological formation and climate, the 

 area around Sydney and on the Blue Mountains to the westward is 

 eminently suitable, though had the latter been 1,000ft. higher the in- 

 fluence of climate would be more pronounced. Here we have a tract 

 of country for the most part covered with Hawkesbury sandstone of 

 the Triassic period, and varying in altitude from sea-level to upwards 

 of 3,600ft. A large proportion of plants are found to persist over a 

 great part of this area without regard to the change of cHmate caused 

 by the increased elevation ; but after the 3,000ft. level is reached, near 

 Wentworth Falls, a falling off of the Sydney plants is noticed, and the 

 dominating influence of climate is further exemplified by the advent of 

 what, in eastern Australia, are known as cold region plants. The com- 

 mon grevilleas which grow about Sydney — such as G. punicea R. Br., 

 G. oleoides Sieb., and G. buxifolia R. Br. — all disappear long before the 



