But In General The Masses Of Known Primitive Rocks, (I Mean Those Which Perfectly Resemble Our Granites, Our Gneiss, And Our Mica-Slates) Are Very Rare In Lavas; The Substances We Commonly Denote By The Name Of Granite, Thrown Out By Vesuvius, Are Mixtures Of Nepheline, Mica, And Pyroxene.

We are ignorant whether these mixtures constitute rocks sui generis placed under granite, and consequently of more ancient date;

Or simply form either intermediate strata on veins, in the interior of the primitive mountains, the tops of which appear at the surface of the globe.) Every thing that tends to enlighten us with respect to the site of the volcanic fire, and the position of rocks subject to its action, is highly interesting to geology.

It is possible, that at the Peak of Teneriffe, the fragments of primitive rocks thrown out by the mouth of the volcano may be less rare than they at present appear to be, and may be heaped together in some ravine, not yet visited by travellers. In fact, at Vesuvius, these same fragments are met with only in one single place, at the Fossa Grande, where they are hidden under a thick layer of ashes. If this ravine had not long ago attracted the attention of naturalists, when masses of granular limestone, and other primitive rocks, were laid bare by the rains, we might have thought them as rare at Vesuvius, as they are, at least in appearance, at the Peak of Teneriffe.

With respect to the fragments of granite, gneiss, and mica-slate, found on the shores of Santa Cruz and Orotava, they were probably brought in ships as ballast.

Enter page number Go  Previous Next
Page 188 of 779
Words from 50985 to 51259 of 211363