220 PATAGONIAN EXPEDITIONS: GEOGRAPHY. 



be traced to certain local fissures and extinct volcanoes, evidences of 

 which may still be observed at frequent intervals throughout the imme- 

 diate region of the basalts. In Fig. 36 there is shown a dike of igneous 

 material with a thickness of fifteen or twenty feet, piercing the stratified 

 Tertiary deposits in the valley of the Rio Chico of the Santa Cruz, some 

 six miles south of Sierra Ventana. I have traced this dike for many miles 

 and found it outcropping wherever the surface of the stratified rocks was 

 not covered over with shingle, thus demonstrating that the covering of 

 shingle was deposited long after the lavas from this fissure had been 

 poured out over the plain. A core of the neck of one of the many extinct 

 volcanoes of this region, with remnants of its crater still persisting at the 

 summit, is shown in Fig. 11. Radiating from this, as from other similar 

 volcanic cores in this region, are one or more basaltic dikes, also piercing 

 sedimentary rocks of Tertiary (Santa Cruzian and Patagonian) age. 



Considering the number and size of these dikes and volcanoes, it does 

 not seem necessary to look farther for the origin of the basalts. And 

 when one travels over the basalt-covered region, it becomes evident that 

 they do not consist of one great sheet, which has flowed out and over a 

 comparatively uniform and gently-sloping surface. There were, on the 

 contrary, many flows from quite different sources, and the various streams 

 were spread over an already deeply-eroded surface. For, from the present 

 appearance of many of these lava streams, it is readily apparent that local 

 inequalities in the surface determined the direction of flow, and that, at the 

 time when volcanic activity was greatest in this region, the surface was 

 already deeply dissected, and distinct drainage systems were well defined. 

 With the outpouring of the lavas, however, many of the latter were either 

 completely or partially obliterated and new channels had to be excavated. 



As to the age of the basalts, there can be little doubt that it was at the close 

 of that period which witnessed the deposition of the Santa Cruzian beds, 

 and which by most authorities is placed as late Miocene, that the volcanic 

 eruptions and other similar phenomena were most active throughout this 

 region. It is true that there are evidences of eruption at both earlier and 

 later times, some having occurred in comparatively very recent times. As 

 examples of these, Figs. 30 and 37 are cited. In the first, the dark band 

 in the center of the picture on the opposite side of the canon represents a 

 remnant of a lava flow which descended this canon when it had been 

 eroded only to the depth marked by the bottom of the small remnant of 



