322 THE STRUCTURE AND SUCCESSION AT 



series, bub wliieh had almost totally ceased during the formation 

 of the Grit, were resumed. They continue throughout the 

 deposition of the Coal Measures proper ; but on the whole, 

 greater elevations were attained here than in the other series, 

 hence greater periods of growth of terrestrial vegetation. 



COAL MEASURES. 



Alternations of strata. — The alternations of sandstone and 

 shale have already been commented upon. Since the rocks often 

 contain erect trees and undistorted ferns, they must have been 

 deposited in shallower but quieter water than those of the 

 preceding series. The presence of small shells, mostly deter- 

 mined to be fresh water species, and the absence of purely 

 marine limestone fossils, all go to show periods of vegetable 

 growth and sediment deposited in inland swamps and lagoons. 

 The beds of coal and shale often being covered with sandstone, 

 however, may show subsidence or floodings at frequent intervals. 

 Mr. Brown reports finding rill marks, rain marks, and foot- 

 prints of land animals near Lloyd's cove and in shale overlying 

 coal. These discoveries, with their distribution, prove the prox- 

 imity of ancient shore lines. 



All these beds in turn have had their period of depression 

 beneath the sea, where the pressure of the ocean and the over- 

 lying strata has brought about those changes necessar}' to form 

 solid shale, sandstone and coal. At last, however, they again 

 were forced up by the orogenic activities which gave rise to 

 the Coxheath and Boisdale hills, and became part of the great 

 compound Sydne}^ basin to the east. 



DYNAMIC CHANGES. 



Uplift against pre-Camhrian. — To the southwest of the 

 Sydney district as a whole are two mountain cores of pre- 

 Cambrian rocks. One of these forms the Boisdale hills, the 

 other the Coxheath hills. Both were uplifted at the close of 

 tiie Carboniferous. In some portions of the province, as Cum- 



