64 The Geology of the Portland Promontory, 



extent the underlying clialk limestone ; thus the exposure 

 of the latter at this spot is to be accounted for. 



Although the chalk-limestone dips under foot at the 

 Portland lighthouse, and is not again visible until the 

 South Australian border is approached, yet there is evidence 

 of its continuity. It must outcrop in the sea bed, not far 

 beyond low water mark, and that frequently, for flints 

 derived from it are plentiful on the beach as far as I went, 

 viz., up to the east end of Discovery Bay. At Danger Point 

 I found, thrown up on the lava rocks, a block of this chalk, 

 about 20 lbs. in weight. The mass was clasped by the 

 roots of a thick fucus (macrocystis pyrifera). Probably the 

 seaweed had been violently torn up in some storm, and being 

 very tough, it had wrenched the block of chalk, on which it 

 had grown, out of its sea bed, and then had, by its great 

 buoyancy, floated it ashore. Again at Bridge water, there is a 

 stratum of pure white colour, which forms a most conspicuous 

 undulating stripe along the clifi' face, for it is sandwiched 

 in between ash-beds of a dark brown or bufl" colour. It is 

 about five inches thick. It appeared to be the ejected 

 powdery debris of a chalk substratum, that had been drilled 

 through as the vent of the volcano was being formed. 

 There can be little doubt but that the rock occurs there 

 at a comparatively shallow depth. 



Investigations by the He v. Julian Woods and Professor 

 Tait, into the fossils of these formations, and of their exten- 

 sions along the coast and elsewhere, place the oyster beds in 

 Upper, and the foraminiferous limestone in the Lower the 

 Muri-avian series — respectively the equivalents of the 

 Upper and Lower Miocene. 



IV. — The Yolcanic Rocks. 



Rocks of volcanic origin cover a large portion of the 

 Promontory. The most considerable accumulation occurs 

 along the south-west shore of Portland Bay. It extends 

 from Cape Grant to the Narrawong siding ; thence it turns 

 to the north-west, or inland ; its thinned edge crosses the 

 raihvay line near the nine mile post ; it overlaps Bat's ridges 

 on the north side of them ; and then it dips under the 

 falsebedded limestone at the Black Gully on the Bridge- 

 water road. Probably there are outliers of this much eroded 

 rock outside of this area, just as there are small patches of 

 the underlying limestone exposed within it. 



