1846.] DARWIN ON THE FALKLAND ISLANDS. 269 



the extra-tropical zones together and the inter-tropical zones to- 

 gether. Hence, from the distribution of existing mollusca, we ought 

 not to feel surprised at the fossil species of the same period, in the 

 most distant quarters of the same great zones, being sometimes iden- 

 tical, or differing only by specific characters. It is however right to 

 add, that not only all the existing shells of the Falkland Islands and 

 of Tierra del Fuego are specifically difi'erent from those of the 

 northern hemisphere, but I think that they differ more palpably in 

 form than do the palaeozoic species from the same quarters : in this 

 comparison however of the living shells, the littoral species are in- 

 cluded ; and these no doubt always show the effects of climate and 

 other external influences more plainly than deep-water genera, such 

 as probably were Spirifer and Orthis. 



The low clay-slate and sandstone districts of the Falkland Islands 

 are broken by numerous ranges varying in height from a few hundred 

 feet to between 2000 and 2500 feet, and all composed of stratified 

 quartz. This rock varies from an arenaceous mixture to a pure white 

 granulo-crystalline mass; it sometimes contains minute imperfect scales 

 of mica arranged in parallel planes, and often small specks of a white 

 substance, like earthy feldspar, exhaling an aluminous smell, but 

 quite infusible under the blowpipe. Occasionally the rock assumes 

 a curious brecciated appearance (apparently of concretionary origin), 

 in which angular fragments of nearly pure quartz are imbedded in 

 an opake siliceous paste, partly formed of the white earthy matter. 

 I have observed these white and yellowish earthy specks in the 

 quartz rocks of several other countries, and likewise in a calcareous 

 rock in one of the Cape Verd islands, produced by the flowing of 

 submarine lava over a recent shelly mass. The rock in this latter 

 case is compact ; and in a series of specimens, the gradual separation 

 of tlie little specks of earthy matter, either through their mutual 

 attraction, or more probably by the segregating infiuence of the 

 stronger attraction of the atoms of carbonate of lime, could be most 

 distinctly traced. There is good evidence that the quartz of the 

 Falkland Islands has been softened by heat ; and the analogy is so 

 perfect between the little earthy specks in the two cases, that I be- 

 lieve they have been similarly produced. 



I nowhere actually saw the superposition of the clay-slate* on the 

 quartz, but in several places on the sea-shore I traced the most gra- 

 dual transitions between these two widely difi'erent formations. It 

 was particularly curious to observe how insensibly the gently inclined 

 planes of stratification in the quartz disappeared, and the highly in- 

 clined cleavage-laminae of the clay-slate, extending in their usual 

 course, appeared : it was impossible to point out where the strati- 



* Captain Sulivan seems to have found on the western island subordinate beds 

 of a conglomerate or coarse grauwacke. On this island there appear also to be 

 traces of tertiary and boulder formations, corresponding with those of Tierra del 

 Fuego. Captain Sulivan observed on the western island numerous basaltic dikes. 



