CORRESPONDENCE. 



455 



In another chapter Davilla records the following fossils from 

 Canstadt, therefore probably from the same beds as his Ornitholite : — 

 molar teeth of Rhinoceros, " line canine, blanche, a petites herborisa- 

 tions bleuatres, adherente a une portion de machoire pareillement 

 herborisee;" another tooth, " Arborisee . . . son interieur est calcine 

 et adhere fortement a la langue ; " six other teeth, " Seavoir, deux 

 arborisees, de Canstadt, Tune de Rhinoceros,* 1'autre inconnue, f 

 deux simplement fossiles, dont une de Cheval dans son alveole. Plus, 

 une portion de machoire aussi fossile, trouvee a Canstadt," and 

 fifteen other bones unnamed. These memoranda would lead one to 

 refer the Canstadt Ornitholite to the Pleistocene age. 



(To be continued.) 



CORRESPONDENCE. 



On the Causes of Earthquakes. 



Sir, — The occurrence of an earthquake of unusual violence in England 

 has naturally given rise to very general inquiries and speculations on the 

 nature and causes of these phenomena ; and various theories more or less 

 ingenious, but, for the most part, vague and obscure, have been broached 

 upon the subject in the different organs of public instruction. 



I have not, however, seen anywhere a notice of that simple, and to my 

 mind conclusive, view of the question which I have put forward in my re- 

 cent work on Volcanos, and I therefore venture to call your attention 

 and that of your readers to it. 



I there refer (p. 294) " these sensible vibratory undulations of the earth's 

 surface to the snap and jar occasioned by a sudden and violent rupture of 

 solid rock-masses at a greater or less depth, and probably the instanta- 

 neous injection into the fissures so formed of intumescent molten matter 

 from beneath." I am happy to find an almost complete accordance be- 

 tween this view and that offered by yourself in your last number. But 

 when you go on to suggest the sudden crystallization of vast underlying 

 masses of mineral matter as the probable cause of such ruptures in the 

 overlying rocks, I must part company with you. Why should we resort 

 to imaginary hypotheses to explain that which, if nature is consistent with 

 herself, is to be simply accounted for by well-known facts ? It is certain that 

 the greater number of volcanic eruptions are preceded or accompanied by 

 local earthquakes, evidently caused by the splitting and rending of the 

 rocks that form the flanks of the volcano, or the surrounding area, by the 

 expansion and rise of intumescent lava beneath. 



In these cases an actual escape of condensed vapour and molten mineral 

 matter does generally take place. Not so, it is true, in the case of other 

 earthquakes, which are usually (not always) felt over more extended areas, 

 and therefore probably proceed from a focus of disturbance more deeply 

 seated, so as not to admit of any outward discharge of erupted matter, 

 gaseous or fluid, but which, nevertheless, are undistinguishable from the 

 former in their seismic phenomena, and therefore may well be believed to 



* Scheuchzer, Physiq. Sacree, pi. 48, fig. 18. f lb., fig. 18, altera. 



