Chapter 18

islands were covered with fine trees; in the conglomerate, I found one 15 feet in circumference perfectly silicified to the very centre. The alternations of compact crystalline rocks (I cannot doubt subaqueous lavas), and sedimentary beds, now upheaved fractured and indurated, form the main range of the Andes. The formation was produced at the time when ammonites, gryphites, oysters, Pecten, Mytilus, etc., etc., lived. In the central parts of Chili the structure of the lower beds is rendered very obscure by the metamorphic action which has rendered even the coarsest conglomerates porphyritic. The Cordilleras of the Andes so worthy of admiration from the grandeur of their dimensions, rise in dignity when it is considered that since the period of ammonites, they have formed a marked feature in the geography of the globe. The geology of these mountains pleased me in one respect; when reading Lyell, it had always struck me that if the crust of the world goes on changing in a circle, there ought to be somewhere found formations which, having the age of the great European Secondary beds, should possess the structure of Tertiary rocks or those formed amidst islands and in limited basins. Now the alternations of lava and coarse sediment which form the upper parts of the Andes, correspond exactly to what would accumulate under such circumstances. In consequence of this, I can only very roughly separate into three divisions the varying strata (perhaps 8,000 feet thick) which compose these mountains. I am afraid you will tell me to learn my ABC to know quartz from feldspar before I indulge in such speculations. I lately got hold of a report on M. Dessalines D'Orbigny's labours in S. America (7/2. "Voyage dans l'Amerique Meridionale, etc." (A. Dessalines D'Orbigny).); I experienced rather a debasing degree of vexation to find he has described the Geology of the Pampas, and that I have had some hard riding for nothing, it was however gratifying that my conclusions are the same, as far as I can collect, with his results. It is also capital that the whole of Bolivia will be described. I hope to be able to connect his geology of that country with mine of Chili. After leaving Copiapo, we touched at Iquique. I visited but do not quite understand the position of the nitrate of soda beds. Here in Peru, from the state of anarchy, I can make no expedition.

I hear from home, that my brother is going to send me a box with books, and a letter from you. It is very unfortunate that I cannot receive this before we reach Sydney, even if it ever gets safely so far. I shall not have another opportunity for many months of again writing to you. Will you have the charity to send me one more letter (as soon as this reaches you) directed to the C. of Good Hope. Your letters besides affording me the greatest delight always give me a fresh stimulus for exertion. Excuse this geological prosy letter, and farewell till you hear from me at Sydney, and see me in the autumn of 1836.