BY THE EEY. W. B. CLARKE, M.A., F.G.S., &c. 279 



associated gaseous vapours, as bread does in an oven, and so 

 becoming in a subaerial position very like some lavas. Filtration, 

 secretion, or other allied operations, fill the cavities afterwards 

 with minerals which are observed not to be confused, but to be 

 deposited one over another. In this state they become what is 

 called amygdaloidal. 



Much of such cellular basaltic lava distinguishes the trappean 

 overflows in Victoria ; and it is also common in parts of New 

 South Wales. It certainly forms the youngest basalt of the 

 gold-fields. Indeed, so recent is it that grasses, reeds and other 

 vegetables are found under the basalt scarcely altered, only 

 scorched and not burned. How is this to be accounted for, if 

 the transmutations by basalt are really effected by intense heat ? 

 In all probability the traps act frequently like granite ; but there 

 are differences also. 



The most striking instances of the slightly changed character 

 of vegetable matter in contact with trap are to be found near 

 Daylesford, in Victoria, where leaves and plants only partly 

 altered, or mineralised in sulphuret of iron, are embedded in fissile 

 clay beneath the basalt, and when examined present in fact 

 almost a recent appearance. Whatever may have been the actual 

 cause of such change, it is impossible to believe that the basalt was 

 at fusion heat, when it overflowed. At Wentworth Gold Field, 

 New South Wales, similar facts are noticed. 



In this colony, the presence of trap and basalt has produced 

 numerous and greater changes at the contact with coal beds and 

 with calcareous and silicated formations. The instances are too 

 numerous to be quoted on this occasion. But all must not be 

 passed over. 



In order to deal with the coal beds, we have first to consider 

 what are the actual effects of heat on combustible substances. 



The effects in the laboratory are well known to Professor 

 Smith and other gentlemen present ; and the artificial manu- 

 facture of coke is known to produce results which imitate the 

 phenomena observed in nature. 



Combustibles may be classed as turf, lignite, bituminous coal, 

 and anthracite or stone coal. These form a series which 

 advances to graphite or plumbago commonly called black lead. 



