190 



Professor Howcliin to be due to subsidence, and this may be the 

 case. If a true dip, it would indicate a higher cone at Blue' 

 Lake than is shown in figs. 7 and 8. 



Summing up the evidence given by dip, we have a western 

 crater (Crater Lake) and an eastern crater (Blue Lake), with 

 a doubtful central area where an additional crater or craters 

 may or may not have existed. 



(ii.) Slopes of External Faces.— As will be seen from 

 fig. 6, the whole of the lower outer slopes of the cones remains-- 

 intact, with the exception of the depression in the north-west, 

 and a small subsidence on the slope below the Hospital (not 

 shown on the map). Before using the evidence of the 

 exterior slopes to assist in reconstructing the cones it is neces- 

 sary to establish two things: — 1. That cones of volcanic- 

 ejectamenta are characteristically symmetrical about the 

 centre of eruption. 2. That at Mount Gambier the present 

 external slopes, where undisturbed, are practically co- 

 incident with the dip of the beds. 



With regard to the symmetry of cinder cones, it is* 

 scarcely necessary to point out that this feature is one of the 

 most consistent of all land forms. The neighbouring cone of 

 Mount Schank, and the various cinder cones of related age 

 in Victoria, are notably symmetrical. Most of the material 

 of the Mount Gambier cones was of such a size as to be 

 little affected by the prevalent winds, and the effect of the 

 wind on the finer material would be no more than to somewhat 

 flatten the eastern and south-eastern slopes ; this feature is 

 to be noted in the contour map (fig. 6). There is, therefore,, 

 every reason to assume that the cinder cones at Mount 

 Gambier were symmetrical. 



The coincidence of the slopes of the surface with the 

 underlying stratification of the ash-beds is noticeable in all 

 the cuttings seen. In places around the Mount, where the 

 surface soil has been partly removed, the surface slope is- 

 found to be the same as that of the consolidated tuffs. Other 

 observations support this idea, and it may be fairly claimed 

 that the contour lines, where drawn in firmly in fig. 7 y 

 represent the general outline of the cones as originally built up. 



The reconstruction shown in fig. 7 is based on the prin- 

 ciples outlined in the preceding paragraphs, and on a mass of' 

 minor observations that it is not necessary to detail. The 

 dips and contour lines preserved at the western crater (par- 

 ticularly Tower Point), and the eastern crater (particularly 

 the Blue Lake Look-out), enable us to reconstruct these two- 

 cones, and the lines suggest them to have been 750 ft. and 

 550 ft. high respectively. The Blue Lake cone may have 

 been higher. 



