230 Notes on Post Tertiary Strata 



by small vessels, and just as on the cliffs of the adjoining 

 cape, the lower portions consist of basalt, and the upper of 

 limestone. 



The mode in which the volcanic rocks of this neighbour-^ 

 hood occur is of special interest, as it enables us to note 

 the relative age, not only of the dune limestone, but also of 

 the underlying sedimentary strata. At the Whaler's Bluff, 

 Portland, three separate formations are exposed in section — 

 viz., the coralline (miocene) at the bottom, next the oyster 

 beds, and jfinally the basalt at the top. Here the dune 

 limestone is absent, but as the coast is traced round to the 

 west, it soon appears, crowning the lava on the cliffs at Cape 

 Grant. In no case are the igneous rocks covered by any 

 other deposit, and it is, therefore, the most recent of all. 



The extinct crater from which the fiery stream proceeded 

 must have been situated somewhere near the sea margin, 

 either on the land, or at a very short distance from it. 

 Skirting Discovery Bay is a series of low cones, extending 

 from Mount Vandyke on tlie north to Mount Richmond on 

 the south. Now, Cape Bridgewater is itself a volcanic hill, 

 and is, moreover, the most southerly extension of the same 

 line of disturbance, its distance from the last-mentioned 

 elevation being only six or seven miles. An examination 

 of the cliffs at this promontory will prove, I believe, that 

 here we have the source of most of the basalt underlying 

 the dune limestone. The highest point of the cape is 460 

 feet above sea level, and is found at about a mile from its 

 extremity, close to the cliffs overlooking Bridgewater Bay. 

 By walking along at the base of these, on what is called the 

 " flat rock," and looking upwards, we see the internal struc- 

 ture of what is evidently an old volcanic cone completely 

 disclosed. The lowest portions, including the rock on which 

 we stand, consist of stratified volcanic ash, of a greyish 

 brown colour, with angular pieces of basalt, varying in size 

 from minute grains up to fragments as large as a man's 

 head, thickly scattered through it, the whole being cemented 

 together so firmly as to form a tolerably hard stone. Above 

 it is covered by a layer of rop}^, scoriaceous lava, which has 

 welled out of the old crater after the first discharge of 

 lapilli and dust. In places, the ash deposit is almost hori- 

 zontal, but towards one particular spot it shows a gradually 

 increasing inclination up to a maximum of about 30 degs. 

 Just here a vertical dyke of lava, not more than a foot thick, 

 intrudes completely through it, and joins the once molten 



