National Geographic : 1897 Nov
PATAGONIA additional evidence in favor of my view * that the great trans verse valleys of Patagonia were in existence prior to the last submergence of this region in the Pliocene, and during which submergence the marine Cape Fairweather beds were deposited. During the elevation that caused the close of this submergence there was distributed over this region the great Bowlder or Shingle formation (Tehuelche formation of Ameghino) of Pata gonia. These benches along the watercourses are not merely river terraces formed of alluvial materials, but are composed of the original strata constituting the Santa Cruz, Supra-Patagonian, and Patagonian beds, as shown in numerous exposures. They are often many miles in width, and I think show conclusively that throughout certain periods during the elevation of this re gion these valleys formed deep embayments into which extended the waters of the Atlantic. Some of the more important of these valleys may even have formed straits connecting the Pacific and Atlantic oceans, as has been claimed by Darwin. Another prominent feature over the Patagonian plains is the occurrence of numerous volcanic cones, appearing usually in groups and at places remote from the Cordilleras. These craters, although now extinct, have been active during comparatively re cent times, as is evidenced by the numerous small lava streams to be found in many places, and which are seen to have flowed directly from some one of these craters down over the sides and into the valleys of the present smaller watercourses, where they have adapted themselves to the curves of the valleys and the inequalities in the surface of the bottoms of the latter, and do not extend into strata forming the sides of the valleys. Such lava streams of comparatively recent origin always present an irregular, hummocky surface, with numerous caverns, and are composed almost always of very vesicular material. A splendid example of such a lava stream may be seen in a small canon on the southern side of the Rio Chico bf the Gallegos river about two miles below Palli Aike, near the point where the present na tional boundary line crosses the Chico. Probably these small volcanoes were active throughout a considerable period in Ter tiary times, and largely furnished the materials of the Santa Cruz beds. That they were active during the depositions of the Santa Cruz beds is evidenced by the occurrence of lavas included be tween successive strata of those beds, which, owing to the ab sence of disturbance in the latter, can hardly be considered as intrusive. *See "On the Geology of Southern Patagonia," Am. Jour. Sci., Nov., 1897, pp. 327-354.