189 



Jacob, of the South Australian Lands Department, in 1910, 

 was made the basis of the contour map. Numerous traverses 

 were made around and across the area, wherever possible, 

 taking two sets of aneroid observations by separate observers, 

 and checking results at all intersections. The contour lines 

 are plotted at 50-ft. intervals. 



It is not claimed that this is a correct contour map of 

 the locality, but in all essential features it is sufficiently 

 correct for the purposes for which it was prepared. It has 

 been found of great value in working out the reconstruction 

 of the cones prior to subsidence. 



(c) Reconstruction of the Chide?' Cones. — In attempting 

 this reconstruction, two lines of supporting evidence are avail- 

 able : (i.) the dip of the tuff beds where undisturbed ; (ii.) the 

 external contours where unbroken by the collapse. 



It is therefore necessary to eliminate the areas disturbed 

 by the subsidences. As already stated, these subsidences or 

 collapses have led to the disappearance of almost the whole of 

 the volcanic cones; they are arranged in the same linear 

 direction as that of the Mount itself, and are shown in fig. 7, 

 where the areas of most abrupt and definite collapse are darkly 

 shaded and the surrounding areas affected by subsequent 

 slumping and land slipping are lightly shaded. The portions 

 unaffected (unshaded) are therefore those where we must 

 obtain our evidence of dip and contour. 



(i.) Dip of Tuff Beds. — Excellent exposures of dip are 

 available at the following places : — The great faces on each 

 side of the "buttress" that runs from the Tower to the margin 

 of Crater Lake; the summit of the Sugarloaf (an isolated 

 point north-west of the Tower) ; the northern and eastern faces 

 of Valley Lake; practically the whole of the abrupt cliff face, 

 averaging 250 ft. in height, that surrounds the Blue Lake; 

 and a small section at the rear of the ornamental tower 

 opposite Gordon's Monument. 



The evidence of the dips in the western portion of the 

 area is distinctly in favour of a centre of eruption somewhere 

 about the centre of Crater Lake. In the eastern portion, 

 around the cliffs of the Blue Lake, the dip is everywhere 

 outwards from the centre of the lake, pointing to a centre of 

 eruption there. 



The evidence of dip about the central area is somewhat 

 non-committal. Unfortunately there is no evidence of dip 

 in the interior of the Leg of Mutton Lake, and the dips 

 visible on the surfaces in the neighbourhood of this lake may 

 be due either to the action of surface wash, or to lisdit ash 

 •deposits subsequent to the formation of the present slopes. 

 The section opposite Gordon's Monument is believed by 



