F. Afneghmo — Geology of Argentina. 17 



have been commenced at a most ancient period, since between the 

 boidders and the Santa Cruz Formation there are no intercalated 

 sedimentary deposits of any kind. 



The only dif&culty which presented itself to me was, as to how 

 the waters which descend from the Cordillei-as to the Atlantic had 

 been able to spread this boulder deposit in so uniform a manner, 

 since the idea of marine action was excluded by the absence of 

 marine remains in it. In any case, it will be a surprise to all, as 

 it has been to me, to know that the Patagonian boulders are 

 a marine formation, as Darwin originally said, but without any 

 relation to glacial agencies and of a geological period much more 

 remote than that to whicb all were more or less disposed to attribute 

 it. On this subject Carlos Ameghino writes to me in a recent letter 

 as follows : — 



" Another of the facts also definitely ascertained on this journey, 

 is the determination that the Tehuelche Formation, or that of the 

 Patagonian boulders, is really a sediment of marine origin, as 

 Darwin originally said, for now I have met with direct positive 

 proofs such as the marine molluscs of that period are, with the 

 only difference that, instead of being a formation of a comparatively 

 recent period, as has hitherto been believed, it will have to ,be 

 relegated to a much more I'emote period anterior to the Pampean 

 Formation, because the species of Ostrea which it contains is of 

 the same type and dimensions as the Ostrea Bourgeoisi of the Santa 

 Cruz Formation. The reason why these deposits of fossil mollusca 

 have hitherto escaped notice by all the explorers of Patagonia is, 

 because they are completely isolated in the formation, and it is only 

 in surveying an enormous superficial extent of the formation that 

 there are chances of finding some of them by accident. These de- 

 posits occupy the summits of certain outliers of plateaux which have 

 remained isolated and only rise a few metres above the general level 

 of the surrounding plain, so that it appears as if at one time these 

 fossils were distributed over all the remainder of the formation, but 

 have only been preserved until to-day in these favoured spots and 

 owing to causes which it has not hitherto been possible to explain to 

 my satisfaction." 



According to this, the boulders were deposited at the bottom of 

 the sea, and over them there extended at other periods a vast 

 formation of marine shells, of which there only remain diminished 

 traces at certain definite spots. There was, then, after the epoch 

 of the Santa Cruz Formation, a submergence of Patagonia, during 

 which the sea penetrated as far as the foot of the Cordilleras ; the 

 definite emergence of those territories is posterior to the ejjoch of 

 the boulder formation and of the banks of marine shells already 

 mentioned superposed upon it. On the other hand, as the oysters 

 of these banks are of large size and of a species similar to that 

 characterizing the Santa Cruz Formation, there is every probability 

 that the boulder formation is Miocene. 



With this there certainly disappear the illusions of those who 

 claim in some way to connect the boulder formation with a great 



DECADE IV. VOL. IV. NO. I. 2 



