NORTH SYDN'EV AND SYDNEY MIXES, C. R. DeWOLFE. 319 



was never sufficiently elevated to form inland seas, where 

 gypsum could b3 deposited as a result of evaporation. Oa the 

 whole, the sulphuric acid theory would seem more probable. 



A third method of orypsum formation is sometimes attributed 

 (according to W. P. Blake— Trans. Am. Inst. Min. Eng., 1901, 

 p. 715), to collections on dry arid surfaces after the manner of the 

 caliche. Since the structure here, however, points to totally 

 different conditions from those belonging to such formations, we 

 may dismiss the subject. 



Whether the gypsum was deposited by the concentration or 

 sulphuric acid method, clearly it indicates, like the limestone, a 

 temporary exclusion of the clay supply. Later this supply was 

 renewed, forming the next layer of marl ; which, however, was 

 somewhat irregular, as proved by the stringers of bedded 

 gypsum running through it. 



The changes outlined above succeeded each other alternately 

 for long periods, since lime, shale, marl, and sandstone alternate 

 until an additional thickness of two hundred feet have accumu- 

 lated. That it is all calcareous, indicates its deposition under sea 

 water. Ripple marks and rain prints occur at diflerent levels, 

 while material in some cases is uniform for some considerable 

 depth, proving slow subsidence. The occasional sudden 

 change from lime to sand or shale, proves intermittent sudden 

 changes of depositional conditions. This is further evident 

 from ripple marks at a contact of fine shale and coarser sand- 

 stone in the railway cutting previousl}' mentioned. The fine 

 shale was deposited in comparativel}^ deep water, or on a mud 

 flat. With a sudden elevation, however, sand was laid down on 

 the shore, for the ripple marks are in the first layer of sand 

 overlying the shale. 



At length these o.scillatory movements ceased, and for a vei-y 

 long time slow and continual subsidence prevailed, without any 

 intervening elevation. The depression was jnst i-apid enough 

 to allow accumulation to keep pace with the sinking sea bottom, 

 so that shore or shallow water deposits are continuous for great 



