xlix 



ARE ALL TBE COLOSSAL IGNEOUS 

 CAPS OF THE TASMANIAN TIERS 

 AND OF THE LOFTY MOUNTAIN 

 PLATEAUX TRUE SILLS ? 



By R. M. Johnston, f.i..s., f.s.s. 



Professor T. W. Edgeworth David, B.A., 

 F.G.S., one of Australia's most distin- 

 guished geologists, has kindly sent me a 

 copy of an interesting paper read by him 

 before the Royal Society of New fc'outh 

 Wales in the year 1893, regarding the 

 occurrence of " Sill Structure '' in the 

 eruptive rocks of that colony. 



It has additional iaterest for Tasmanian 

 geologists, inasmuch as it raises the 

 question at the head of this paper, viz. — 

 " Are all the Colossal Igceous Laps of the 

 Tasmanian Tiers, and of the Lofty Moun- 

 tain Plateaux — such as Ben Lomond, Mount 

 Wellington, and the Great Plateaux of the 

 Lake Country— True Sills ? " 



Igneous Sheets or Sills. 



Sir Archibald Geikie gives a very graphic 

 d*^scription of the nature and chaiacter- 

 istic structure of an Igneous Sill as 

 follows : — (Text Book of Geology, pp. 573— 

 576.) 



"Eruptive masses have been intruded 

 between other rocks and now appear as 

 more or less regularly defined beds. In 

 many cases it will be found that these 

 intrusions have taken the place between 

 the planes of stratification, The ascending 

 molten matter, after breaking across the 

 rocks, or rather, after ascending through 

 fissures either previously formed or opened 

 at the time of the outburst, has at last 

 found its path of least resistance, to lie 

 along the bedding planes of the strata. 

 Accordingly it has thrust itself between 

 the beds, raising up the overlying mass 

 and solidifying as a nearly or exactly par- 

 allel cake, sheet, or sill. It is evident that 

 one of these intercalated sheets must pre- 

 sent such points of resemblance to a sub- 

 aerial stream of lava as to make it occasion- 

 ally a somewhat difficult matter to deter- 

 mine its true character, more especially 

 when, owing to extensive denudation or 

 other cause, only a small portion of the 

 rock can now be seen." 



So far we have a very clear definition of 

 the manner in A-'hich true igneous sills 

 have been formed, and their mode of in- 

 trusion along the weaker planes and 

 fissures of strata beneath the surface. In 

 Tasmania there are abundant illustrations 

 of clearly defined massive sills open to 



inspection, especially so, along the precipi- 

 tous walls of the coast line between Black- 

 man's Bay and Cape Frederick Henry. 



About 14 years ago I drew the attention 

 of the members of this Society (I) to 

 one of these remarkable sills exposed on the 

 coast line near Blackman's B^y, intercalated 

 between thestrat'fiei beds of permo - carb. 

 mudstones and limestones ; and in my large 

 work on "Toe Geology of Tasmania, I 

 again described this sill or intercilated 

 igneous sheet — illustrated by enlarged 

 drawings of sections (2) — in the following 

 words : — 



•In various places along the coast line 

 of the Lower Derwent many natural 

 sections occur where th:j fossiliferous mud- 

 stones (apparently) unaltered at point 

 of contact, repose quietly in horizontal 

 beds which naturally fill up the uneven 

 surface of the underlying older greenstone. 

 A sketch of a very tine section is given 

 showing this relation for several miles 

 between Blackman's Bay and Passage 

 Point. One section in part'cular not only 

 shows in an unmistakable manner that the 

 fossiliferous mudstones are more recant (3) 

 than the main mass of the older greenstone 

 upon which they rest, but that boDh are 

 o:der than a minor dyke or soeet of green- 

 stone of a somewhat similar character to 

 the older. This intrusive greenstone after 

 bursting verticiUy through the older 

 greenstone (basal sill), and the lower beds 

 of sedimentary limestone, suddenly hends 

 hack ond forms a sheet about seven feet thicic, 

 runmng parallel and intercalated hetwe'.n the 

 stratified planes of the marine mudstones. 

 Fine sections showing the same relation- 

 ship also occur for miles continuously 

 between Passage Point and Adventure 

 Bay.'' 



It is not surprising, therefore, that some 

 years later (i892) Proftssor David, Captain 

 Hutton, and others, to whom I had per- 



(1) Proc. Roy. Soc. Tas., 1885, pp. 343 

 360, 410 ; xUd , 1686, pp. 18-26, illustrated 

 by a number of platts and diagrams. 



(2) Geology of Tas., p. 102. Plate show- 

 ing position of intercalated sill. 



(3) Two or three years ago in an address 

 delivered by me to the Members of the 

 Mining Institute of Australia, which met 

 at Hobart, I stated that I bad reason to 

 alter my opinion in regard to the age of the 

 older gieenstone at this place, and now re- 

 gard it also as an older sili thrust of colossal 

 dimensions underlying the mudstone series 

 at this point, but of later age. 



