450 Dr. H. J. Johnston-Lavis—On the form of Vesuvius, ete. 
Before quitting this part of the subject, there are two observations 
to make. In the first place, my critic continually makes me say 
that the Atrio was formed by the explosive eruption known as the 
Plinian. Now, this is displeasing to me, since I look upon that 
eruption as of comparatively feeble type, and also because I have 
stated the direct converse. My words are (op. cit. p. 36), ‘The 
eruptions that excavated the great crater of the Atrio, and subse- 
quently piled up the cone of Vesuvius,” and I devote nearly 100 pages 
to describe these eruptions; and again (p. 87, first paragraph), “It 
(Braeislak’s theory) presupposes only one paroxysmal eruption, as 
the cause of all this great cavity, whereas a number did the work 
piecemeal.” Again, a similar statement is made on p. 65, second 
paragraph, p. 68, fifth paragraph, and particular attention should be 
paid to p. 82, fourth paragraph, where I refer the great crater in 
large part to as far back as the explosive eruption of Phase VI. 
period 1. 
I take this opportunity of mentioning an observation I have made 
since I wrote my paper, namely, that the slope of the Vesuvian 
cone is steeper on the N.E. than on the 8.W., as if its axis were 
undergoing gradual displacement in that direction, that 1s to say, the 
reverse of what occurred in pre-Plinian and Plinian times. May 
not the chimney of the volcano not having a straight course, result 
in this eccentricity consequent upon its being gradually worn so by 
the constant transmission of fluids through it? 
Having to the author’s own satisfaction demolished my theory, he » 
proceeds to replace it by one of his own which we will now examine. 
His theory is that the prevalent wet wind being the Scirrocco, or South 
wind, that impinging on the southern crater ridge has demolished 
it, leaving the northern intact, and quotes to support his statement 
the difference in the denudation going on upon the Kasia chain of 
Sikkim, where in a distance of 20 miles the difference of rainfall is 
as 1 to 84. Surely the author would not compare an isolated peak 
with a high range of mountains. The distance between the two 
ridges of Somma is not more than four kilometres, yet, we are 
to believe that the one part of Somma for a height of 500 metres 
has been swept away, leaving the other portion intact. Moreover, 
the author fails to take into consideration that for any amount of 
rainfall to produce a denuding effect, it must be collected together 
on an impermeable surface, from which it must acquire momentum 
before it can exert appreciable erosive action. Another fact put 
forward to support the author’s view is that one side of the tower 
of Notre Dame of Paris is much more weathered than the other, 
due to prevalent winds. The author would have done better had he 
been silent upon this, for in the one, we have the result of weather- 
ing due to frost, wetting and drying and dissolving the stone, and the 
other to true surface erosion. If we take the loss of a few milli- 
metres from the surface of Notre Dame during some centuries of . 
exposure of a comparatively soft stone, and then estimate how long 
a considerable ridge of lavas, scorias, etc., 500 metres high, would 
require to be denuded at the same rate, we would find that this 
