Note 31 — Chapter XIII

I will here put together all the (apparent?) exceptions which I have met with to the supposed law that erratic blocks are absent in the intertropical regions of the world. First, in the Bulletin de la Société Géologique, 1837, p. 234, there is an account of some erratic blocks near Macao (lat. 22° N.); but as it is distinctly stated they are all of granite, and the greater number of even the same coloured variety, as the granitic rock, on which they rest, the case need not be considered. Secondly, in a late number of the Madras Journal, Dr. Benza has described some erratic blocks lying on a plain between the Neilgherries (lat. 12° N.) and Madras. He states that the foundation-rock of the country is gneiss, “while the granite clusters are more elevated, and affect either a prismatic form, or are piled up one on the other, like logging stones.” Dr. Benza had the kindness to inform me that these masses are very large, and that several are piled one upon the other. Again, Brongniart says (Tableau de Terrains, p. 83), “On cite aussi dans l’Inde, au pays d’Hyderabad (lat. 17° N.), des blocs énormes de granite, amoncelés les uns sur les autres” (Deluc neveu). Every one must draw his own conclusions from these accounts, regarding the probability of erratic blocks being heaped up, one upon the other, like logging stones. The same doubt likewise partly applies to the Macao case. With respect to the boulders of Hyderabad, Dr. T. Christie has distinctly stated (Edin. New Phil. Jour., Oct. 1828, p. 102), that they are in situ, and has explained their origin. For my own part, I cannot forget that whole granitic hills at the Cape of Good Hope, which, from weathering, have assumed a boulder-like form, were once described as transported masses. The two next cases do not properly come under consideration, for they refer to masses lying in the valleys of lofty mountains. We must not overlook such accidents as bursting of lakes, earthquakes, and the action of former coast-lines. Helms, in his Travels (English translation, p. 45), states he was astonished to find the highest snow-capped mountains near Potosi (20° N.) covered with a stratum of rounded granitic stones. He supposes they must have come from Tucuman, which is several hundred miles distant; yet at p. 55 he says, at Iocalla (a few leagues only from Potosi), “a mass of granite many miles in length, rises in huge weather-beaten rocks:” the whole account is to me quite unintelligible. Lastly, M. Gay (Annales des Sciences, 1833) describes granitic boulders within the valley of Cauquenes (lat. 33°-34° S.), in the Cordillera. I visited this place: the boulders and pebbles are not large, and those beyond the mouth of the valley are small. The case did not appear to me nearly so extraordinary as it seems to have struck M. Gay. I cannot agree with his assertion that this rock is not found in that part of the Cordillera: but this is a subject which I shall discuss in a future work.