u 



years. In the space of 2^ millions of years 

 it follows that rock of a uniform depth of 

 833 feet has been swept away from the tops 

 of all our higher greenstone tiers and 

 mountain plateaux. 



This is not at all an exagsrerated estim- 

 ate of the amount of rock-waste, whose mass 

 originally covered the present gieenstone 

 masses. These greenstones at the points now 

 exposed to waste, are proved by the valu- 

 able microscopic investigation of Messrs. 

 Twelvetrees and Petterd. to be of such 

 composition and crystalline texture (sill 

 structure), as to have required the pressure 

 of an immense saperincumbent mass of 

 rock ; and great slowness in coo'.ing; 

 to induce that original character and 

 successive forms of crystallisation which 

 the gentlemen named have been able to 

 determine as sill structure. 



From such considerations we may follow 

 with coDfiience the general conclusions 

 arrived at by Messrs. Twelvetrees and 

 Petterd. 



Messrs. Twelvetrees and Petterd, whose 

 prior investigations are recorded in the 

 Proceedings of this Society and elsewhere, 

 in their latest observations '■ On JVJesozoic 

 Dolerite and Diaba'-e in Tasmania," 

 have now, shown how extensive these 

 iavestigations are. They have examined 

 carefully numerous microscopical sections 

 from all parts of Tasmania, and they 

 modestly state that such observations are 

 mefely regardel by t hem as "stepping 

 stones to more complete knowledge.'' 



The general coriclusion formed by thf m 

 as regards the nature and mode of origin of 

 our greens' one rocks still remaining un- 

 wastedby denudation are, that: — " They were 

 never in the form of a lava overspreading 

 the land in the presence of the atmosphere. 

 They have been undeniably produced by 

 the crystallisation of a magma which was 

 injected or intruded into strata lying 

 below the surface. They have not crystal- 

 lised rapidly, but under the pressure of 

 euperincumbent rocks, which we seemed 

 compelled to believe have been carr ed 

 away hy subsequent denudation. There is 

 absolutely nothing to show that they ever 

 succeeded in establishing communicatiori 

 with the surface. If, however, they did, 

 both the pipes by which i he magma « scended 

 and the basaltic flows in which that 

 ascent finally resulted, have been wasted 

 without leaving a trace tehind. The entire 

 absence of mesozoic basalts in the island 

 suggests that these dolentes always were 

 subterranean, and that the faces and cliffs 



which we now see are subterranean sections 

 lifted for our inspectioa by one or other of 

 the earth movements, which geological 

 science so often reports." 



I quite accept the conclusions of Messrs. 

 Twelvetrees and Petterd that the sections 

 examined by them (which must in the case 

 of the caps of the mountains, regarding 

 subsequent denudation, be from levels 

 from 1000 to 3000 feet below the original 

 surface) (1) Were never in the form of a. 

 lava overspreading the land in the presence 

 of the atmosphere ; (2) That they have not 

 crystallized rapidly while under the pres- 

 sure of superincumbent rocks, which have 

 subsequently been wasted by long con- 

 tinued denudation. 



What thooght occurs to me at this stage 

 is— (I) Could a massive sill, 2000 to 3000 

 feet thick, be thrust for vast distances 

 between the planes of stratified bedding — ■ 

 say within 800 feet of the surface — without 

 causing inniimerable fissures and fractures 

 through which some portion s of the magma 

 would be forced to the surface in the form 

 of lava, ashes, etc. ? To me it seems in- 

 credible at present (2) Supposing 

 also, that by gigantic fissure eruptions a 

 tide of lava welled upwards to the surface, 

 and in places attained in its flow a thick- 

 ness of two thousand or more feet before 

 cooling. What, eventually, would charac- 

 terise the more rapidly cooling surface 

 from the magma, more slowly cooling, at a 

 depth of from 1000 to 2000 feet below the 

 upper surface of the s»me flow? Would it 

 not be possib'e for the slower cooled maema 

 at gieat depths to show " siiJ structure" 

 as regard crystallisation ? 



It must be remembered that at a depth 

 of 3000 feet from the surface the pressure 

 from a superincumbent mass would be 

 equal to the weight of 240 atmospheres, 

 i.e., the pressure at the surface, and at a 

 depth of SOOO feet respectively, wou dbeas 

 1 is to 2iO. The rate of radiation of heat 

 from a cooling mass, from surface to base, 

 wovild, at the same time, proceed in an 

 inverse ratio. 



Geikie states that " In former geological 

 ages extensive eruptions of lava, without 

 the accompaniment of scoriae, with hardly 

 any fragmentary material?, and with, at 

 the most, only flat, dome- shaped cones at 

 the points of emission which have taken 

 place over wide areas, from scattered rents 

 along lines or systems of fissures. Vast 

 sheets of lava have in this manner been 

 poured out to a depth of many hundred 

 feet, completely burying the previous sur- 



