CLASSIFICATION. 3 



For convenience of classifying sedimentary rocks, the 

 deposits are arranged in groups or eras ; systems or periods ; 

 series or epochs ; stages or ages, and beds. The second term in 

 each case relates to time. Thus we speak of the Mesozoic 

 group, Triassie system, Hawkesbury series, Narrabeen stage, 

 and Estheria bed. 



When one speaks of coal measures it is intended to in- 

 clude not only the coal seams of that series, but also those 

 beds closely related to the seams, which consist chiefly of 

 shales and sandstones of fresh water origin. 



The word seam is applied to a bed of coal. It need not 

 necessarily be confined to clean coal, but also includes any 



Fig. 3. Phyllotheca australis. 



small bands of stone deposited about the same time, and inter- 

 bedded in the coal. When two seams occur so close together 

 that they may both be extracted from the same workings, they 

 are known as "twin seams." Coals occur from Palaeozoic to 

 Tertiary times, but certain periods appear to have been more 

 favourable to the formation of coal than others. For instance, 

 in Australia, the Permo-Carboniferous period is that in which 

 our most productive fields of bituminous coal occur, while the 

 brown coal is found in Tertiary strata. 



