236 PROCEEDINGS OP SECTION C. 



western slope is steep and scoriaceous, dipping at from 20° 

 to 30° while the eastern slope is very gradual. Eruptions 

 of lava appear to have occurred at intervals along the ridge. 

 The trend of the ridge may correspond with that of some 

 deep-seated fissure, along the whole length of which the 

 lava has boiled up, so that no craters have been formed, 

 except at points along the line, where the volcanic energy 

 became centralized, as at " The Gap" ; or, more probably, 

 the ridge has been formed by the progressive shifting of 

 the volcanic orifice along the line of fissure, in which case 

 each of the small lava sheets, which take their rise from the 

 ridge, may mark the situations which were occupied from 

 time to time by the "Gap" crater, before it came to rest 

 in its present position. 



CAVERNS. 



A singular structure, locally met with, in the laterite, is 

 that of circular cavernous spaces, shaped like inverted saucers, 

 or comparable to immense bubble hollows.* One of these caverns 

 was met with by Mr. H. Jew in sinking his 220 feet shaft in 

 Reynold's selection in Portion 512, Parish of Scone, near Emma- 

 ville.* " AVhen sinking through the hard laterite, Mr. Jew found 

 the ground sound hollow, and as the sinking continued, he broke 

 into a natural cavern at 30 feet from the surface, the base of the 

 laterite being separated from the underlying surface of rotten lava 

 by a space of two and a-half feet. The floor of this hollow was 

 strewn with small round concretions of brown iron ore which had 

 fallen out of the laterite from the roof. The cave extended on all 

 sides as far as Mr. Jew could see, but was not explored for any 

 distance away from the shaft. This hollow is now concealed by 

 wooden slabbing, A similar cavern exists in Portion 1, Parish of 

 Arvid, where the volcanic tuff", five to six feet tliick, forms the 

 spring of a low arch from two to three feet high and about fifteen 

 yards wide. The cave has been laid bare through the partial 

 collapse of the roof. These caves have probably been formed by 

 the lava sinking from under the tuff" beds, which may have been 

 close to points of eruption, so that the lava would have a tendency 

 to be forced to a higher level here than elsewhere ; and so, as the 

 eruption subsided, would, like water, seek to find its own level, 

 and thus leave a hollow between its surface and the harder crust 

 of overlying tuff". Darwin, in his ' Geological Observations,'! 

 describes similar hollows at the bases of small basaltic craters at 

 Chatham Island in the Galapagos Archipelago, where he observed 

 several ' nearly circular pits with perpendicular sides from twenty 

 to forty feet deep,' formed, he thinks, by the falling in of the 



♦Geology of the Vegetable Creek Tin-mining Field, p. 32. 

 t2nd Edition, p. lib. 



