ORIGIN AND AGE OF THE SHINGLE FORMATION. 221 



lava just mentioned. The second picture shows a still more recent flow. 

 Here there appears, only a few feet above the water of the same stream, 

 a layer of basalt some twelve feet thick, showing a decidedly columnar 

 structure below, though less distinctly above. This thin sheet of lava 

 rests on a bed of coarse shingle, the bottom of which is, at present, but 

 just above the level of the water in the stream. 



Origin and Age of the Patagonian Shingle Formation. The discus- 

 sion of these problems, as also those relating to the age and origin of the 

 basalts, are more properly geological than geographical in their nature, 

 and they will be treated more in detail in that portion of these volumes 

 devoted to the geology of this region. They have been such important 

 factors, however, in determining many of the chief topographic features 

 of the Patagonian plains, that a few remarks as to their age and origin 

 would seem not to be out of place. 



Save over some of the basalt-covered areas, the plains of Patagonia are 

 everywhere covered to a depth of several feet with an accumulation of 

 rounded and water-worn pebbles and bowlders. With an average thick- 

 ness of perhaps thirty feet, this deposit of coarse gravel has come generally 

 to be known as the Shingle Formation. Where exposed in sections, it is 

 seen to consist throughout of a heterogeneous mass of water-worn stones, 

 with but a very slight admixture of sand and clay. The materials are 

 generally unstratified and appear as a bed of coarse, loose conglomerate 

 unconformably overlying the older sedimentary rocks beneath. It occurs 

 alike on the pampas and in the valleys, as well as over the slopes of the 

 latter. It is wanting over the surface of the higher of the basalt-covered 

 tablelands of the interior, and throughout a considerable portion of the 

 lava beds in the vicinity of the Gallegos River it has been covered over 

 and concealed by lavas of more recent origin than the shingle. Although 

 the shingle is continuous throughout the entire region extending from the 

 coast to the Andes, save over the higher basaltic tablelands, it increases 

 in thickness and its materials become coarser, as one proceeds inland from 

 the coast toward the base of the Andes. 



In the Narrative I have already mentioned the discovery, near Cape 

 Fairweather, at the base of the Shingle formation, of marine deposits of 

 late Pliocene age. L moreover, mentioned the significant fact that I 

 found these same marine deposits in the bluffs south of the eastern 

 end of Lake Pueyrredon, at an altitude of five thousand feet above the 



