952 



SCIENCE. 



[N. S. Vol. XV. No. 389. 



These are the only evidences yielded by the 

 Sierra Costa range similar to those usually 

 depended on in the Mississippi basin to es- 

 tablish a dissected peneplain, and they may 

 be deceptive, for it is not certain that the com- 

 parative uniformity in the height of the peaks 

 may not be due to the intersection of slopes 

 in accordance with the theory advanced by 

 Penck. A symmetrical drainage system 

 nearly everywhere trenched down to the late 

 Neocene base level might be expected to reduce 

 all the principal divides to about the same 

 level. 



But there is another and stronger evidence 

 of peneplanation at the level of the high 

 peaks. It is to be found in the behavior of 

 the streams. In a general way the rivers and 

 creeks of the Sierra Costa region ignore the 

 structure. For instance, the old Coffee Creek 

 rose in the Abrams mica schist (not very re- 

 sistant relatively to other formations) flowed 

 obliquely across on to the Salmon hornblende 

 schist (quite resistant), made a sharp turn 

 and then crossed at nearly a right angle belts 

 of mica schist (not resistant), serpentine 

 (moderately resistant), mica schist (not re- 

 sistant), serpentine (moderately resistant), 

 gabbro (very resistant), serpentine (less re- 

 sistant), granodiorite (moderately resistant) 

 and serpentine (less resistant). Why was not 

 the stream deflected along the softer belts and 

 around the massifs of gabbro and granodiorite 

 if the structure in any way controlled the 

 course ? 



All the higher peaks in this region are com- 

 posed of granitic rocks, gabbro or hornblende 

 schist, showing that these three are the most 

 resistant to weathering. All the valleys nar- 

 row decidedly upon entering on the area of 

 the gabbro and the hornblende schist showing 

 that these formations are the most resistant 

 to stream erosion. Yet the streams will cross 

 these formations when they might take an 

 easier course around them. 



There is another way of looking at it : The 

 granodiorite batholiths were the last to be 

 intruded. They have the form of gigantic 

 volcanic necks, being in most cases vertical 

 columns of granitic rock rising \\p through 

 the other formations. Whether or not any 



of the material ever reached the surface and 

 formed rhyolite volcanoes, it is likely that the 

 strata were more or less arched over these 

 massifs. The bulging must have been 

 effected so rapidly that any important streams 

 flowing over their sites would be deflected. 

 Without a subsequent rearrangement of the 

 drainage system, the trunk streams should 

 avoid these granite massifs, which in some 

 important eases they do not. There can be 

 little doubt that the drainage system of the 

 Sierra Costa area is superimposed on the 

 structure. 



The independence of the Sierra Costa 

 streams from the structure was already de- 

 veloped when the drainage was no lower rel- 

 ative to the rocks than the tops- of the pres- 

 ent peaks. It implies that a rearrangement 

 had occurred previous to the beginning of 

 trenching of the present valleys. Such a Re- 

 arrangement must have been efl^ected on a 

 plain. When Inigration of streams is brought 

 about during simple down-cutting or deepen- 

 ing of valleys it is controlled by the structure. 

 A radical rearrangement independent of 

 structure necessitates a plain, either of ag- 

 gradation or of denudation. 



This argument does not establish the connec- 

 tion between such a plain and the uniformity 

 in height of the present peaks. We do not 

 know whether the rearrangement occurred on 

 an uplifted sea bottom (a plain of aggrada- 

 tion) or on a true baselevel of erosion. And 

 if the latter, we do not know whether this 

 baselevel was developed in the plane of the 

 summits of the present peaks or higher in the 

 strata. These are problems to be solved in the 

 future. At present we can only say that the 

 examination of the stream courses indicates 

 that such a peneplain was developed, increas- 

 ing the probability that the present summits of 

 the Sierra Costa peaks do represent a dis- 

 sected peneplain. 



Very few geologists have climbed to the 

 summit of the Sierra Costa Mountains. Dr. 

 A. C Lawson had a partial view of the sup- 

 posed peneplain level from Battle Mountain, 

 altitude about 7,675 feet above the sea. He 

 recognized the pertinence .of the evidence and 

 was willing to accept the idea of the dissected 



