OF ATACAMA AND OOQUIMBO. 281 



the shades and modifications <>f which it w.mld IMJ both difficult and useless to describe. How- 

 there is one variety \\hich perhaps merits ^articular attention, a* it appears often in the 

 vicinity of metallic v. I thin in a green porphyry in white felspar Oil bite), which passes 



int.' mi-it-'. 



As far as the eastern slope of the cordilleras of the coast, the entire Pacific shore is composed 

 of rocks belonging to this formation. Here begins a secondary stratified formation of an epoch 

 anterior to tin- upheaval of the Andes, under which, and the tertiary strata of the Great Basin, 

 the first dips, to burst out again in many places of the loftier ridge, of whose very summits it 

 not unfrequently forms a considerable portion. As has been intimated, the characteristics of 

 the rocks of the primary formation are a crystalline structure, and absence of stratification. 

 Examining the different portions of the group attentively, we may divide them into two classes: 

 First, masses of granite that form the lower part of the system, which we usually detect at the 

 greatest distance from the centre of the Andes. Their essential element is orthose felspar, 

 sometimes passing into gneiss or mica-schist. And, second, granitic and porphyritic masses, 

 almost always containing albite and amphibole, in contact with the secondary stratified forma- 

 tion they have upraised. 



The rocks embraced in class first are generally sterile, easy to decompose, and probably are 

 but the debris of a primitive formation, or, to speak more definitively, of a formation anterior 

 to the epoch of the secondary, and which preceded the elevation of the Andes. Class second, on the 

 contrary, consists of proper rocks of sublevation, which contain immense numbers of metallic veins, 

 often found near their contact with the very formation they dislodge. They are very rarely en- 

 countered close to the shore and level of the sea ; and, as might be inferred from the difference in 

 structure of the mountains north and south of Chacabuco, their development varies with the 

 two sections of country. In the province of Atacama this formation generally reaches within 

 20 to 25 miles of the sea, with an elevation above it of 1,500 feet; in the southern provinces it 

 is not found within twice that distance, nor until a great altitude is attained, sometimes near 

 the very summits of the Andes themselves. From the organic remains found in it, it probably 

 belongs to the epoch of the Jurassic or cretaceous formation. Though poor in calcareous and 

 arenaceous rocks, it abounds in porphyries, alternating "with porphyritic schist, breccia, porphy- 

 ritic tufa, and different compact, schistoidal, siliceous rocks of unknown nature. 



In the north, where they appear at different altitudes, and often in the first line of escarp- 

 ments of this formation, calcareous strata and compact fossiliferous rocks are quite frequent ; 

 but south of Chacabuco they are only found near the summits of the Cordilleras, and beyond the 

 valley of the Maypu they disappear entirely. Thus, in the latitude of Copiapo we find them 

 at very varied elevations ; in that of Coquimbo, not far from its base ; and in the parallel of San- 

 tiago, only in the superior portion of the formation over the Portillo Pass. They seem not to 

 form a distinct layer in the group, but merely to be subordinate to the stratified porphyry and 

 compact schistose or breccoidal rocks principally constituting the chain of the Andes, and 

 which belong to an epoch anterior to its elevation. This is the secondary formation. The 

 tertiary, whose epoch is posterior to the sublevation, is composed of horizontal or slightly 

 inclined strata, comprising the great longitudinal valley and a part of the small plains on 

 the coast old water-courses, which appear as so many terraces or steps above one another, 

 attesting epochs in the slow and continued rising of the coast subsequent to the brusque 

 upheaval of the Andes. Not that I suppose this great chain to have been elevated by the 

 tension of the fluidified rock at any single epoch, but simply desire to indicate the distinctive 

 effects or results of plutonic and aqueous forces. There is no doubt that Dr. Darwin's views 

 have received the sanction of geologists, and he has shown that its structure can only be 

 explained on the supposition that the rent strata composing its axis were repeatedly injected 

 after intervals sufficiently long to allow the upper parts or wedges to become cool. That at 

 least a portion of the chain Las encountered both agents, and passed alternate eras above and 

 beneath the ocean, is proved by silicified trees, from three to five feet in circumference, having 

 36 



