350 
THE GEOLOGIST. 
racters denote that it bears a great resemblance to Pelagonite. The 
metamorphism of argillaceous rocks into jasper is very frequent, and 
must not be regarded as a silicatization, for analysis shows that the 
jasper thus produced contains, in most cases, less silica than the rock 
from which it has been formed The presence of alkalies in an argil- 
laceous rock seems to have facilitated its metamorphic transformation. 
Hence argilolite has been more easily transformed than day. 
The minerals formed by metamorphism, added to the changes pro- 
duced by artificial heat on clay rocks, show that during metamorphic 
action the temperature was tolerably moderate, and that water must 
have played as great a part as the contact of the eruptive rock. 
We promised, in our last paper, to refer again to M. Castelnau's 
observations on the earthquakes of the Cape of Good Hope. 
On the 14th of August, 1857, at half-past eleven at night, Cape-towa 
experienced a violent shock, which was followed by two others of ten 
seconds' duration each. 'Ihe whole phenomenon lasted about forty 
minutes, or rather longer, especially near the Table Mountain. The 
usual subterranean thunder was heard at the same time. Domestic 
animals appeared as frightened as the men.* Luckily, at the Obser- 
vatory none of the instruments were disturbed. 
This earthquake was felt 200 miles to the north, and 400 miles to 
the east of the Cape. The direction of the undulations seems to have 
been that of the meridian. The ships stationed in Table-bay all felt 
the effects of the shock, and the " Solertia," a vessel commanded by 
Captain Boisse, which, at the time of the earthquake, was one hundred 
miles south of Cape-point, was launched out of its course to the 
eastward. 
The same observer informs us that these phenomena have been 
comparatively rare at the Cape of Good Hope. Since the commence- 
ment of the present century, we have only the following on record : — 
In the year 1809, a succession of shocks lasted from the 4th of December 
to the 24th of the same month. lu 1811, an earthquake, felt on the 
2nd of June, was described by Burchell, who. says that a shock was 
also experienced in the preceding year. In 1843 a very slight motion 
of the earth's crust was observed. The most formidable of these, and 
'■^ See The Geologist for July, 1858, p. 300. In reference to Boussingault's 
observations, recorded in this number, we will quote here Alexander von Humboldt's 
■words on the same subject : — " To man," says this illustrious author, " the earth- 
quake conveys an idea of some universal and unlimited danger .... We 
are accustomed from early childhood to draw a contrast between the mobility of 
water and the immobility of the soil on which we tread ; and this feeling is con- 
firmed by the evidence of the senses. When, therefore, we suddenly feel the 
ground move beneatli us, a mysterious and natural force, with which we are 
previously unacquainted, is revealed to us as an active disturbance of stability. 
A moment dL'stmys the illusion of a whole life; our deceptive faith in the repose 
of nature vanishes, and we feel transported, as it were, into a realm of unknown 
destructive forces. Every sound, the faintest motion in the air, arrests our 
attention, and we no longer trust the ground on which we stand. Animals, and 
especially dogs and swine, participate in the same anxious disquietude; and even 
the crocodiles of the Orinoco, which at other times are as dumb as our little 
lizards, leave the trembling bed of the river, and run with loud cries into the 
adjacent forests." — T. L. P. 
