104 SECTION AT ONE TREE POINT, 



R.S. TAS. 



"surrounding stratified rocks. " Now, though I carefully 

 searched for fragments of the Palseozodc mudstone, I failed 

 to find them. On the other hand, fragments of schistose 

 rocks, which do not occur anywhere near, are most numer- 

 ous. This seems to indicate that the magma only broke 

 through beds belonging to the Prc-Cambrian schists (11)- 

 Probably the explosions preceding the eruption completely 

 shattered the overlying Palaeozoic strata, and when eventu- 

 ally the eruption of the magma took place it only filled 

 the cauldron-like cavity produced by the explosions, en- 

 closing and enveloping only rocks from greater depths. 

 The examination of these inclusions is another problem 

 that awaits solution, and probably much information as t# 

 the coimposition of the strata in greater depth may be 

 gleaned from it. 



Another feature of the Basalt is its absence of coluni- 

 nar structure (PI. vii.) ; instead of the customarv columns 

 the Basalt is massive, and rather inclined to part in hori- 

 zontal layers- This peculiarity makes it appear as if there 

 had been several eruptions producing different flows. Of 

 these I have not found any evidence; the Basalt presents 

 the appearance of one prciduced by a single eruption. 

 Sometimes it shews a vesicular appearance, particularly in 

 contact with the underlying strata. But such a layer of 

 vesicular Basalt right in the middle of the massive one is 

 also seen at the southern end of the section. I attribute 

 it to a particularly strong percentage of water in this par- 

 ticular parti of the magma, but it must not be taken a^s 

 proof of several eruptions (12). Even a casual examina- 

 tion proves that enormous masses have been removed, and 

 that what we see now forms only a small portion of the 

 original mass. The Basalt eruption at One Tree Point 

 was therefore not a fissure eruption, but is produced in 

 harmony Mnth other occurrences by a single eruption of a 

 cone-like mass, of which now a small portion along its 

 western edge is preserved. (PI. viii.). 



Messrs. McLeod and White made a chemical and 

 microscopical examination of this Basalt, and have proved 

 the complete absence of magnesia. They have further 

 shown that the small red grains represent the red variety 

 of Olivine, Fayalite ; Augite, according to the authors, is 



(11) This would indicate that in tho Derwent Valley the Permian 

 series most probably rests directly on Pre-Cambrian Schists. 



(12) In tlie great Basalt stream flowing from the plateau of the 

 Dscholan down the valley of the Yaniiuk numerous layers of vesicular 

 Basalt can be observed in the massive ])ortions. These layers, which 

 start as suddenly as they die out, are. in my opinion, due to local 

 development of steam. 



