xlvii 



behind. The eiith^e absence of niesozoic 

 basalts in the island suggests that these 

 dolerites always were subterranean, and 

 that the faces and cliffs now seen are sub- 

 terranean sections, lifted for inspection by- 

 one or other of the earth movements 

 which geological science so often reports. 

 The authors discuss the name to be applied 

 to the rock. In Europe and America it is 

 known as diabase, in England mostly as 

 dolerite. European geologists use the 

 term dolerite for the coarse interior parts 

 of thick lava sheets, and consider that 

 rock a local, unimportant modification of 

 basalt. But most English petrographers 

 use the group name dolerite for all rocks 

 intermediate between gabtro and basalt, 

 and confine the term diabase to altered 

 varieties of dolerite. If one or the other 

 term becomes obsolete, it will be necessary 

 to adopt the one which gains general 

 recognition. The tertiary basalts of 

 Circular Head, Table Cape, Lefroy, etc., 

 often exhibit a slightly ophitic structure, 

 and correspond with what goes under the 

 name of dolei-ite in Germany. They may 

 be distinguished from our mesozoic dolerite 

 by their abundant olivine, glassy base, and 

 greater freshness. The discovery of a 

 little auriferous wash in the first and 

 second basins of the South Esk, at Laun- 

 ceston, has led some to believe that the 

 niesozoic dolerite might be gold-bearing ; 

 but the fact is, that the sand obtained 

 carried, besides small flakes and water- 

 worn pellets of gold, grains and crystals of 

 quartz, zircon, sapphire, and ilmenite, all 

 minerals of the granite and slate country 

 in the upper reaches of the river, and 

 must be referred to that source. No useful 

 minerals have yet been found in this rock, 

 and the lodes and reefs of our vjirious 

 mines are all of earlier date. The experi- 

 ence of our miners in this respect has been 

 so uniform, that search for ore deposits 

 in the dolerite is invariably regarded as 

 useless. 



LAVA V. SILL ORIGIN OF THE HIGHER 

 COLOSSAL IGNEOUS MOUNTAIN CAPS. 



Mr. R. M. Johnston, F.L.S., in his obser 

 vations, placed a very, high value upon the 

 petrological investigations of our igneous 

 Tocks by Messrs. Twelvetrees and Petterd. 

 He stated that prior to the time when 

 these gentlemen commenced their splendid 

 microscopic investigations — of which their 

 latest contributions, read that evening, he 

 referred to as " a rich mine of wide and 

 valuable knowledge and logical deduction" 

 — the local general geologists were not 

 possessed of the necessary data to enable 

 them to form satisfactory conclusions. 

 He further stated that, in his opinion, 

 conclusions as to the exact mode of origin 

 •of our massive igneous mountain caps — 



whether as sills or lavas— would be of 

 little scientific value without a thorough 

 survey of all our igneous rocks by such 

 methods of careful systematic examina- 

 tion as are now being carried on by 

 Messrs. Twelvetrees and Petterd. Mr. 

 Johnston said he frankly accepted these 

 gentlemen's conclusions regarding the con- 

 ditions under which, what remains 

 of, our massive igneous rocks of our tiers 

 and mountain plateaux were cooled and 

 crystallised, i.e. 1. The portions still 

 unwanted by denudation were slowly 

 cooled under an immense pressure of super- 

 incumbent r-ocks far below the level of the 

 original surface. 2. The manner in which 

 crystallisation has taken place corresponds 

 exactly to that of true dykes, roots, and 

 sills. He also pointed out that there were 

 abundant examples of true igneous in- 

 tercalated sills nearHobart, between Black- 

 man's Bay and Adventure Bay : but he 

 doubted whether the great massive igneous 

 caps of Ben Lomond, Mount Wellington, 

 and the Lake plateau were ever ejected 

 or intruded as true sills ; i.e., were inter- 

 calated between the planes of bedding 

 of other rocks, without any portion 

 reaching the surface as a lava flow ; 

 although the unwasted portions now re- 

 maining, undoubtedly, show crystal- 

 lisation identical to that of sill structure. 

 He contended that it seemed to him incredi- 

 ble that a massive sill 2,000ft. to 3,000ft. 

 thick could be thrust for vast distances 

 between the planes of stratified bedding, 

 say within 800ft. of the surface, without 

 causing innumerable fissures and frac- 

 tures through which some portions of the 

 magma would be forced to the surface in 

 the form of lava, ashes, etc. He also con- 

 tended, if we assume a period of 2t^ million 

 years to have elapsed, that any massive 

 lava flow, if exposed to surface waste or 

 denudation at the normal rate of 1ft. in 

 every 3,000 years, would be uniformly de- 

 nuded to a depth of 833ft. from the original 

 surface. He also pointed out that at a 

 depth of 3,000ft. from the surface the 

 pressure from a superincumbent mass 

 of rock would be equal to the weight of 

 240 atmospheres. Taking these matters into 

 consideration, he then asked :— 1. Suppos- 

 ing that by gigantic fissure eruptions a tide 

 of lava welled upwards to the surface, and 

 in places attained in its flow a thickness 

 of a thousand or more feet before cooling — 

 What eventually would characterise the 

 more rapidly cooling surface magma from 

 the magma more slowly cooling at a depth 

 of from 1,000ft. to 2,000ft. below the upper 

 surface of the same flow ? 2. As the radia- 

 tion of heat from a cooling mass, from sur- 

 face to base,would proceed in an in verse ratio 

 to that of pressure— Would it not be pos- 

 sible for the characteristic crystallisation 

 of sill structure to be produced without 

 the agency of a "true intercalated sill ?" 



