Fue > 23, 1894. 
cidents caused by movements a little more rapid than 
usual, or by the sudden slipping of large masses out of the 
position of unstable equilibrium into which the slow move- 
ments had brought them. If these large or primary 
movements were fault‘movements, one would almost ex- 
pect to find the axis on the edge, and not in the middle 
or at the bottom of a steep trough in the ocean-bed. If 
the primary movement, on the other hand, was principally 
one of revolution about the axis, interrupted by an 
occasional sliding of the mass on one side of the axis upon 
the mass on the other, then we must look for secondary 
movements at some distance from the axis, where the dis- 
placement caused by revolution is naturally greater. Is it 
possible that the lesser shocks were more or less local 
movements of this character? It is curious to notice that 
the total intensity of the series of shocks amounts to 
186,690 absolute units, or about rg times the acceleration 
due to gravity. The large expenditure of energy implied by 
this total suggests at least a possibility of a very appreciable 
amount of movement in the land-mass of ‘Tasmania and 
southeastern Australia. Though, so far as I know, there 
is no evidence of elevation or depression, one does not like 
to think of Mother Earth wasting so much of her strength 
for naught. 
LETTERS TO THE EDITOR. 
¥*, Correspondents are requested to be as brief as possible. The writer’s name is 
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cation will be furnished free to any correspondent. 
The Editor will be glad to publish any queries consonant with the character of the 
journal. 
Volcanic Rocks in the Keewatin. 
In view of the article published in Sczence, No. 571, 
entitled ‘‘ Volcanic Rocks inthe Keewatin of Minnesota,”’ 
and the very numerous recent papers on the same subject, 
viz.: ‘‘ Archean Volcanic Rocks,” it may be interesting 
to your readers, and, in any case, I think it is fair to 
myself, to publish the following letter of mine on the 
same-subject, which was written fourteen years ago. 
Any comments by me are, I think, unnecessary. 
ALFRED R. C. SELWyn, 
Deputy Head and Director, 
Geological Survey Dept., Ottawa. 
‘Montreal, 9 December, 1879. 
““My dear Professor Dana: 
“‘T have just read your remarks' in reference to what 
I have ventured to call the Volcanic Group of the 
Quebec series of Sir W. E. Logan. I should like very 
much to know exactly what your views on this question 
are, and hope at some future time to hear them from 
yourself personally. In the meantime I would make a 
few explanatory remarks on the points you refer to in my 
paper. You say, ‘The evidence of the general volcanic 
origin of the second group is not stated and the kind of 
rocks mentioned make a remarkable assemblage to be 
spoken of as these volcanic rocks.’ ‘This would seem as 
if I had meant to assert that all the rocks mentioned as 
constituting the group were of volcanic origin. I might 
certainly have made the matter plainer had I specified 
those rocks in the group which there were reasons for 
supposing to be of volcanic origin. It never occurred to 
me, however, that in giving a general description of a 
group of strata* of mixed volcanic and ordinary sedimentary 
origin it would be necessary to do so. As regards the 
evidence of a volcanic origin, of some of them I can only 
say now that it is of precisely the same kind as that 
which, in respect: of similar British strata, has been con- 
ld merican Journal af Science, vol. xv., 1879. 
*Now, 1891, appropriately terme d ““pyrocla stic 
S\CIUBIN CIE. 107 
sidered to be conclusive by almost every British geologist 
of note, including De la Beche, Lyell,* Murchison, Sedg- 
wick, Jukes," Scrope and a host of others now living.’ 
Further, that these conclusions, first arrived at by the 
most careful and minute geological investigations and 
mapping of the stratigraphy, have heen, or are supposed 
to be, entirely confirmed by the, comparative ely recent, 
microscopical and chemical investigations of these same 
rocks. 
“Tt is now rather more than thirty years since I took 
an active part, under the geologist I have first named, in 
working out in all their intricate details the great Lower 
Silurian and Cambrian and older volcanic series of north 
Wales. Since then I have had abundant and world-wide 
opportunities of studying volcanic formations of all ages, 
recent, Tertiary, Mesozoic and Paleozoic, and I may say 
that it is on the result of this world-wide geological in- 
vestigation and experience, and not on the occurrence of 
labradorite or any other particular mineral, that I have 
come to the conclusion that we have in Canada, as in 
Great Britain and elsewhere, good evidence of the exist- 
ence of volcanic strata, and consequently of volcanoes, in 
Silurian or Cambrian and pre-Cambrian epochs. I am 
quite aware that most of the peculiar rocks, which, in 
common with a majority of British and some American 
geologists, I hold to be of volcanic origin, have heretofore 
been generally, and doubtless quite correctly, described 
simply as ‘crystalline,’ ‘metamorphic,’ or ‘igneous’ 
rocks. But this, it seems to me, does not refer so much 
to the question of their origin, as it does to that of their 
present condition and character, and if we carefully study 
their stratigraphical relations in the field, and their 
microscopic and physical characters, we at once find—at 
least, such has been my experience—that some other 
explanation of their origin and associations is required 
besides that of their being ordinary sedimentary deposits 
in a metamorphic condition. Indeed, your own and Mr. 
Hawe’s careful and admirable investigations of the 
chloritic formation in the New Haven region seem to 
me to demonstrate the entire probability, to say the 
least, of the igneous and volcanic origin of the rocks you 
describe. It is, I believe, generally admitted that rocks 
having the mineral and physical peculiarities characteristic 
of many volcanic products would be more easily affected 
by metamorphic agencies, especially hydration, than 
those which are of ordinary and unmixed sedimentary 
origin and that these old volcanic rocks should have 
assumed these metamorphic or altered characters is, of 
course, what might be expected and their having done so 
certainly does not negative the supposition of their 
volcanic origin. It seems to me that @ przorz probabilities 
of the existence of volcanoes in Bozoie and Palaeozoic 
epochs are very strong and that those who oppose any 
such idea should be prepared, like those who hold the 
Opposite opinion, to state some good reason for their 
views, and also the particular geological epoch when, in 
their opinion, volcanic outbursts first occurred. If, on 
the other hand, the existence of volcanoes in these early 
geological epochs i is admitted, then we may very naturally 
expect to find their products associated with the ordinar y 
sedimentary rocks of the period, in the same manner as 
we do those of the volcanoes of recent and Tertiary times. 
And this is what British geologists generally claim to 
have done. Ihave no wish to dogmatise on this question 
and only desire the truth, whatever that may be; but at 
present I cannot help feeling that if I am in error, Iam 
so in very excellent company, and that the views of such 
eminent geologists as I have named, and based, as I know 
” 6th ed., pp. 605, 603. 
3Vide Lyell's ° ‘Elements of Geolo 
s ‘Manual,’ 2nd ed 
p. 
“Memoirs,” Geological Suryey of Great Britain, vol. ii , chap. 5 
24. 
5trde Ramsay, 
