350 



THE GEOLOGIST. 



racters denote that it bears a great resemblance to Pelagonite. Tli© 

 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 clay. 



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 -town 

 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. In 1811, an earthquake, felt on the 

 2nd of June, was described by Burchell, who saj^s 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. 800. 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 beneath us, a mysterious and natural force, with which we are 

 previously unacquainted, is revealed to us as an active disturbance of stability. 

 A moment destroys 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. 



