BY LOFTUS HILLS, M.B.E., M.SC. 127 



complete at the time the Epi-Cambro-Ordovician plutonics 

 reached their viise-en-place. 



The close of the Silurian period of sedimentation wit- 

 nessed the geosynclinal collapse and the occurrence of the 

 Epi-Silurian orogenic movements. The trend-lines are re- 

 gularly directed N.N.W., and overfolding occurs both to the 

 East and to the West, overthrusting having taken place on a 

 considerable scale. The movements, although undoubtedly in- 

 tense, did not produce the universal schistose structure which 

 was the result of the preceding diastrophic period. The 

 batholithic phase of this Epi-Silurian orogenic period was 

 one of great importance and size, as it was responsible for 

 the intrusion of our so-called Devonian granites, gabbros, 

 and serpentines, together with their associated congeners. 



Since the cessation of the Epi-Silurian orogenic move- 

 ments there has been no recurrence of compressive forces 

 in Tasmania. The vertical and upwardly directed thrusts 

 of the diabasic invasions at the close of the Mesozoic era 

 apparently gave rise to no horizontal thrusts. This conclu- 

 sion seems inevitable as the result of the recent researches 

 of P. B. Nye, and the evidence is to the effect that this dia- 

 basic upthrust carried upwards to varying heights isolated 

 masses of Permo-Carboniferous and Trias-Jura sediments. 



It has been long recognised that the present configura- 

 tion of Tasmania is very largely due to the effect of tensional 

 faulting on a large scale. During the period under review 

 it cannot be claimed that this conception has been elaborated 

 to any considerable degree. The contention advanced by E. 

 C. Andrews and Dr. Griffith Taylor that the Midlands is a 

 Rift Valley seems to have been disproved by P. B. Nye's 

 recent investigations. 



It is undoubtedly true, however, and a mass of confirma- 

 tory evidence relating thereto has been collected during the 

 last 18 years, that block faulting has taken place parallel 

 to the general trends of the coast-line. The basal beds of the 

 Permo-Carboniferous system have, for example, been found 

 at sea-level, and 3,000 feet above that level, and a great part 

 of this is due to tensional block faulting, but the exact con- 

 tribution to this difference of level by the upthrusting during 

 the diabase injections has not been determined. Minor faults 

 of this tensional series have been recognised, but it cannot 

 be claimed that the major breaks have been accurately 

 located. It is hoped that the work now in progress on our 

 coal-fields will result in some valuable data in connection with 

 this problem. 



