250 THE PHYSICAL MANIFESTATIONS OF WEATHEBmG- 



become separated by thin clayey layers of a white color, more 

 or less striped with ochreous lines. In a more advanced stage 

 of the process large ovoidal masses of basalt (but little changed 

 in appearance excepting the development of a slaty concentric 

 structure) lie in the cliiBf separated by a considerable thickness 

 of the whitish clayey layers, which are stained by irregular 

 oehreous lines At last the centres of the spheroidal masses 

 yield, ard finally the change is so complete that the concentric 

 arrangement is entirely lost, and a soft whitish or yellowish- 

 white argillaceous deposit, with few oehreous spots or lines, 

 takes the place of the compact basalt. 



'*In basalts of more compact structure these changes take 

 place more slowly. The grayish blue basalt in the lUawarra 

 range, near Broughton's Head, when long exposed, is discolored 

 exteriorly to a depth of an inch and a half. The colors, begin- 

 ning within, are dirt-brown, grayish yellow, ochre-yellow, 

 brownish red; and they are evidently dependent mostly on 

 changes in the condition of the iron which the rock or its 

 minerals contain. 



^'When the rock includes much chrysolite, the results of 

 decomposition in some instances give a fissile or micaceous 

 appearance to the rock. At Prospect Hill, five miles west of 

 Paramatta, this change is in progress. The rock is a black 

 ferruginous basalt of homogeneous aspect, breaking with a 

 smooth fracture and no appearance of crystallization. It con- 

 tains chrysolite; but the grains are small and not apparent 

 except on very close examination. . . . 



'^Were we unable to trace the transitions, and distinguish 

 the columnar structure through the whole, we should scarcely 

 suspect its basaltic origin. Indeed, it was pointed out to me 

 as an instance of mica slate overlying basalt. Particles of 

 rusted mica, as they seemed, were distinct, and it much re- 

 sembled a decomposing variety of that rock. On close inspec- 

 tion and an examination of the rock in different stages of 

 change, it became evident that the pseudo-mica was nothing 

 but altered chrysolite, which had rusted from partial decompo- 

 sition, and split into thin cleavage scales. 



''The crystals of chrysolite have evidently a parallel position 

 in the rock, and hence the plane of easiest cleavage lies in the 

 same direction, or, as the cleavage shows, parallel with the 

 upper surface, that is, at right angles with the vertical axis of 



