150 PROCEEDINGS OF SECTION C. 



The period of igneous invasion was followed by a certain 

 amount of faulting, in which the dyke rocks have been to some 

 degree crushed and dislocated. 



There are a few small isolated patches of glacial till which 

 are probably of Permo-Carboniferous age. Any sediments of 

 later date which have covered the area surrounding Zeehan have 

 been completely removed by the long cycle of erosion which has 

 been operative in western Tasmania since the close of the Mesozoic 

 Era.^ The region is an elevated one, in which the maturity of the 

 erosion cycle has been somewhat masked by successive negative 

 movements of the strand-line. 



III. — ^The Igneous Rocks of Devonian Age and the 

 Relationship between them. 



From the uprising magma which made its appearance in almost 

 every part of Tasmania in or about the beginning of the Devonian 

 period, many varieties of non-metallic consolidation products 

 which we call igneous rocks have been born. 



The rise of molten material has certainly not been one massive 

 upwelling of homogenous material from the deeper regions of the 

 earth's mass. There have been at least two distinct periods of 

 irruption, and two great groups of igneous rock types have 

 successively consolidated. 



The two groups may be conveniently referred to as the 

 acidic and basic groups. ^ 



The rocks belonging to the basic group made their appearance 

 in the earlier period of igneous invasion. Those of acidic character 

 followed after them, actually penetrating them in some instances, 

 and almost always producing some contact metamorphic effects 

 upon them. 



Geological evidence in all cases is in favour of the rapid 

 succession of the acidic after the basic irruption. The two types 

 are found together in a number of widely separated localities, and 

 there is a singular lithological resemblance between the members 

 of each type in these localities. Both groups show a marked 

 tendency towards the development of differentiated rock types, but 

 the acidic group is on the whole less characterised by difierences 

 between its component members than is the basic. 



The constant features of association, the constant but small 

 difference of age, and the recurrence of similar rock-types all point 

 to the derivation of the acidic and basic magmas from one mother- 

 magma by the process known as magmatic differentiation. More- 

 over, the variation within each group appears to have arisen from 

 a continuation of the operation of the same process at a period 

 later than that of the first differentiation which produced the groups. 



In the region which is under discussion there is a marked dis- 

 parity between the relative bulks of the acidic and basic types 



1 It is doubtful whether any but the low-lying areas of the western part of Tasmania were 

 protected from denudation by submergence beneath the sea in early Tertiary time. 

 2 Compare E. Suess' Sal and Sima, " The Face of the Earth," Vol. IV., pp. 544, et seq. 



