CHAPTER VIII. NORTHERN CHILE.—CONCLUSION. A Section from Illapel to Combarbala; gypseous formation with silicified wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley; fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas, silicified wood.—Conglomerates, nature of former land, fossils, thickness of strata, great subsidence.—Valley of Despoblado, fossils, tufaceous deposit, complicated dislocations of.—Relations between ancient orifices of eruption and subsequent axes of injection.—Iquique, Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the porphyritic conglomerate and gypseous formations.—Great subsidence with partial elevations during the cretaceo-oolitic period.—On the elevation and structure of the Cordillera.—Recapitulation on the tertiary series.— Relation between movements of subsidence and volcanic action.—Pampean formation.—Recent elevatory movements.—Long-continued volcanic action in the Cordillera.—Conclusion. GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA CRITICAL INTRODUCTION. Of the remarkable “trilogy” constituted by Darwin’s writings which deal with the geology of the “Beagle,” the member which has perhaps attracted least attention, up to the present time is that which treats of the geology of South America. The actual writing of this book appears to have occupied Darwin a shorter period than either of the other volumes of the series; his diary records that the work was accomplished within ten months, namely, between July 1844 and April 1845; but the book was not actually issued till late in the year following, the preface bearing the date “September 1846.” Altogether, as Darwin informs us in his “Autobiography,” the geological books “consumed four and a half years’ steady work,” most of the remainder of the ten years that elapsed between the return of the “Beagle,” and the completion of his geological books being, it is sad to relate, “lost through illness!” Concerning the “Geological Observations on South America,” Darwin wrote to his friend Lyell, as follows:—“My volume will be about 240 pages, dreadfully dull, yet much condensed. I think whenever you have time to look through it, you will think the collection of facts on the elevation of the land and on the formation of terraces pretty good.”