600 CENOZOIC ERA— AGE OF MAMMALS % 



hundred species found there, about fifty are entirely peculiar — being 

 found nowhere else in the world. Of the others, all are characteristic 

 California species. Now for the explanation : During late Tertiary 

 and early Quaternary times the continent was higher than now, and 

 these islands were a part of California. We have already given proof 

 of this fact on page 562. During that time California, including these 

 islands, was occupied by a flora not greatly different from that of the 

 islands now. By the oscillations of the Quaternary the islands were 

 separated. Then came the northern invasion of species, changing 

 some of the native species and destroying others, and forming the Cali- 

 fornia flora of to-day. The islands were spared this invasion by isola- 

 tion. It is probable, therefore, that in the island flora we have a some- 

 what near approach to the flora of California before the invasion.* 



Thus, then, regarding the Cenozoic and the Modern as consecutive 

 eras, and the Quaternary as the transitional, revolutionary, or critical 

 period between, we see a great, and, if we had lost the Quaternary, an 

 apparently sudden, change of species. Yet this change, as great as it 

 is, is not to be compared in magnitude with that which separates the 

 great eras or even ages from each other. Evidently, therefore, we must 

 regard the lost interval between the Archaean and Palaeozoic, and that 

 between the Palaeozoic and Mesozoic, yes, even that between the Meso- 

 zoic and Cenozoic (as small as this latter is in comparison with the 

 others), as all of them far greater than the whole Quaternary period ; 

 or else the forces of evolution must have been far more active in those 

 earlier times than more recently. 



4. Drift in Relation to Gold. — We have already stated (p. 240) that 

 gold occurs in two positions, either in quartz- veins intersecting meta- 

 morphic slates (quartz-mines) or in drift-gravels (placer-mines). The 

 auriferous slates may be of various ages. In the Appalachian chain, 

 and in the Ural Mountains, and in Australia, the slate or schist is 

 metamorphic Silurian. In California it is Jura- Trias. The placer 

 gold deposits are everywhere Quaternary drift-gravels. 



There has been throughout all geological time a progressive con- 

 centration of gold, as well as many other metals, in a more and more 

 available form : 1. It was first disseminated in excessively small quan- 

 tities, too small to be detected, through the slates, derived doubtless 

 from the sea, in the waters of which it is detectable in very small quan- 

 tities. 2. After the upheaval, crumpling, metamorphism, and Assuring 

 of these slates, the gold was dissolved, and accumulated, along with 

 silica and metallic sulphides, in these fissures, as auriferous veins. 3. 

 Atmospheric agencies acting on these outcropping veins dissolved away 

 the sulphides, and left the gold in a still more available form along the 



* American Journal of Science, vol. xxxiv, p. 457, 1387. 



