SILL STRUCTURE AND FOSSILS IN ERUPTIVE ROCKS. 289 
that the gigantic masses of gabbro which are so extensively 
developed along the estuary of the Derwent, as well as along the 
south-east coast, including Freycinet’s Peninsula, are in reality 
sills rather than old lava flows, as was formerly contended by 
some. Their intrusive character had been ably argued for pre- 
viously by Mr. T. Stephens, r.c.s. A subsequent examination 
has convinced me that the bulk of these gabbro rocks, such as 
those which form the fine headlands of Cape Pillar and Cape 
Raoul, as well as Mount Wellington, are sills. The intrusive 
mass at Mount Wellington might perhaps by some be termed a 
laccolite on account of its great size. 
During a recent visit to Tamworth, in company with Mr. 
Donald Porter, I examined several sections near the town and at 
Moore Creek. The intricate way in which granite sills are there 
intercalated between the planes of bedding of the sedimentary 
rocks, if it does not baffle description, certainly baffles mapping. 
A zone of sills about five miles in width girdles the intrusive 
granite. The zone is composed of sedimentary rocks alternating 
with sills. The sedimentary rocks are of Devonian or possibly 
Silurian age, altered at their line of contact with the main boss 
of granite into garnet and chlorite rocks, These pass into an 
outer zone of chiastolitic rock. The latter is succeeded by fine 
grained claystones, converted by the sills into chert and jasper, 
and by the thin radiolarian limestones with the coralline lime- 
Stones of Moore Creek, from one hundred to about 1,000 feet in 
thickness, 
The sills in this outermost zone are from a fraction of an inch 
4p to several yards in thickness, and alternate so regularly with 
the claystones and radiolarian cherts and limestones that it is 
difficult to believe that the eruptive rocks are not interbedded. 
The whole zone for several thousands of feet in thickness is half 
sill half sediment. A careful examination of the sills shows that 
they trespass slightly across the planes of bedding of the sedimen- 
ay rocks, and the latter along their planes of contact with the 
Sills, both above and below, show evidence of contact metamor- 
S—Oct. 7, 1896, 
