132 THE PROGRESS OF GEOLOGlCAi. RESEARCH IN TASMANIA, 



Ordovician orogenic period, was the intrusion of the granite. 

 This granite has been recognised at four localities: — South 

 Darwin, Dove River, Mount Farrell, and Bond's Peak. At 

 the two former localities it is clearly intrusive into other 

 members of the complex, but in the vicinity of Mount Farrell 

 there is a mergence by insensible gradations into members 

 which are clearly extrusive. This very puzzling structure 

 is being dealt with by the writer in his work on the "Metal- 

 logenic Epochs of Tasmania," and it seems probable that we 

 have here a possible illustration of "extrusion by de-roofing,'^ 

 as propounded by R. A. Daly. If this is so, however, the 

 granitic phase of the batholithic period is not confined to 

 the end-point alone. 



Apart from the areal mapping of the basic and acid 

 members of the Epi-Silurian petrogenic period there has 

 been very little progress, with one noticeable exception, to- 

 wards arriving at general conclusions in regard to the struc- 

 ture and relationships of the various igneous massifs. The 

 exception referred to is the paper read before this Associa- 

 tion in 1911 by L. Keith Ward, entitled "The Heemskirk 

 "Massif — its Structure and Relationships." In that paper 

 the conception is developed that the Heemskirk Massif pos- 

 sesses a definite bottom and is chonolithic in character 

 rather than laccolithic or batholithic. Ward further proceeds 

 to hypothecise two parallel lines of crustal weakness along 

 which igneous intrusion has taken place, and maintains that 

 the various Epi-Silurian igneous massifs, although possibly 

 connected in depth along these lines, are elsewhere quite 

 separate intrusive bodies. Work carried out since the pre- 

 paration of that paper, however, throws serious doubt on 

 the accuracy of these conclusions. A great difficulty in re- 

 gard to the acceptance of the existence of the Bischoff and 

 Heemskirk-Middlesex lines of crustal weakness which have 

 been the loci of igneous intrusion lies in the significant fact 

 of the concordance between the orientation of the major axes 

 of the igneous massifs and the Epi-Silurian fold axes. As 

 stated above, the Epi-Silurian trend lines have a bearing of 

 N.N.W. — a direction which is at right angles to that of the 

 two lines indicated by L. K. Ward. Add to this the irregular 

 but wide distribution of the outcrops of both basic and acid 

 massifs of this series, which is obvious from a glance at the 

 Geological Map of Tasmania, and the difficulty of accepting 

 Ward's conclusions is apparent. 



The evidence seems to point to the conclusion that the 

 Epi-Silurian magma reached its final resting place in the 



