PROCEEDINGS OF SECTION C. 389 



Tcrs and Bovlden. — Besides these master joints, less prominent ones, 

 arranged more or less rectangularly, have served to concentrate the 

 atmospheric attack along these directions, producing cuboidal blocks. 

 The attack proceeds most rapidly at the solid angles, where a maximum 

 surface for a given mass is presented, less rapidly at the edges, and 

 scarcely at all on the faces of the blocks. The cuboidal, therefore, 

 gradually approaches the spheroidal shape, and all stages in this process 

 in the production of " tors " and isolated boulders can be seen on the 

 surface in this area. The granite of the You Yangs appears to be very 

 vulnerable to the chemical attack of atmospheric water, and a further 

 stage, resulting in the excavation of an already formed spheroidal mass 

 occurs. Water finding its way to the heart of the mass along minor 

 joints has decomposed the whole mass, and brought some of the iron 

 and silicates into solution. Evaporation from the surface has caused 

 the water charged with salts in solution to be sucked up to the surface 

 of the mass where those salts are left behind as a deposit impregnating 

 and indurating the outer crust and forming a resistant skin to further 

 attack. 



On the under side of the boulder this skin has either not 

 been formed or has been removed, and wind and water erosion 

 has scooped out the inner decomposed granite, leaving a dissected 

 spheroid. 



Rock Basins. — At the south-west end of Station Peak there is 

 marked on the geological map what is described as a large projecting 

 mass of granite. It appears to be an early stage in the production of 

 a gigantic tor. The granite originally surrounding this isolated mass 

 has been removed by denudation, and the sub-circular outline of the 

 mass has been determined by the directions of several joint planes. 

 The highest point of the mass is at an altitude of about 440ft, 

 (aneroid reading), and the southern wall is very steep and about 80ft. 

 high. The outer circumference of the rock mass is about 600yds. 

 The north and north-east edge of the mass is only raised a few feet above 

 the level of the general surface of the granite, and on ascending from 

 this side the most interesting feature of the structure is seen. The 

 central part of the upper surface is seen to be depressed, and consti- 

 tutes a rock basin, partially filled with soil from the decomposition 

 of the granite. The inner rim of this rock basin at its junction with 

 the soil has a circumference of about 150yds. to 180yds., while the 

 maximum and minimum diameters are 50yds. and 40yds. respectively. 

 The basin is comparatively shallow, the outer rim being possibly 10ft. 

 to 12ft. above the central soil cap, while the depth of soil in the centre 

 of the basin may be 2ft. or 3ft. In most granite areas small rock basins, 

 from a few inches to a few feet in diameter, are to be seen, and their 

 origin is not difficult to explain. A minute depression in the granite 

 surface causes rainwater to settle in it, and thus provides the means for 

 the commencement of the disintegration of the granite. Vegetation 

 gets a footing on the granite soil, and its decay provides acids for further 

 chemical attack, and so the hollow is slowly widened and deepened. 

 The interest of this granite rock basin on the south-west flank of the 



