207

tides and currents. To what other causecan we attribute the frequent intermixtureof animals inhabiting the land, with thoseinhabiting the sea? of wood, fern, bones oflacertae, moluscae dwelling only in shal-lows, with pentacrini dwelling only in thedeeps ?

If partial deposition tended, in someplaces, to increase subsisting inequalitiesof surface, in others it would tend to dimi-nish them: it would tend to diminish themwherever the depositing fluid experiencedinterruption from ridges previously exist-ing; that is, practically speaking, in allthose situations where we find on one sideof a mountain chain a different series ofrocks from that which we find upon theother. 11

Thus porphyry is very common on theItalian side of the Alps, rising betweenBolzano and Brixen to the height of 4000feet, but on the German side it is altoge-

* See Journal de Physique, tom. xlix. p. 212. Seealso Voyages de Saussure, § 981. and Nicholson’sJournal-, vol. iv. p. 264.

* O 8