Page:The Journal of geology (IA journalofgeology11893univers).pdf/343
depth of fifteen feet or more with clays through which are mingled boulders of diorite and granite and fragments of quartz. Further east, at a lower level, some of the clays have been washed over and contain subangular fragments of quartz, some of them two feet in diameter, many of which are somewhat water-worn. It is perhaps worth mentioning that these water-worn quartz fragments imbedded in clays were regarded by Hartt as the best evidence of glaciation. They were finally eliminated as such evidence near the end of a rainy season by my finding a land-slide filling up a small ravine in which the bed of the stream had been strewn with similar quartz fragments, and the whole buried beneath a slide of crumpled clays. A highly instructive lesson can be had on the subject of boulders and clays, their origin and relations to the so-called drift of Brazil from Professor Derby's paper on nephelene rocks in Brazil.[1] Anyone reading that article can readily fancy how Professor Agassiz, in a flying trip across São Paulo and Minas, would have interpreted these clays and boulders of different kinds and different colors.
In regard to the so-called erratics I should mention also the opinion of another observer and writer upon Brazilian geology. Emmanuel Liais, formerly director of the Imperial Observatory at Rio de Janeiro, is very positive that there are no evidences whatever of glaciation in Brazil. Of the boulders supposed to be erratics, he says:[2]
"These boulders though numerous are always in the immediate neighborhood of the veins from which they are derived.....Though presenting sometimes the appearance of erratics by their abundance and rectilinear arrangement, they are not transported boulders, and have nothing in common with erratic phenomena.....I have not been able to find any signs of the existence of a boulder that can be regarded as erratic and coming from a region distant from the one where it is found. In the vicinity of these isolated boulders one always finds dikes, veins or simply masses or boulders of the same material intercalated with the terrain in place."
He speaks of the occurrence of dikes of diorite from which many of the boulders cited by Agassiz have been derived. More