Lawson.] 



Post-Pliocene Diastrophism. 



•37 



been formed, should have been completely obliterated, while those of 

 San Pedro Hill and San Clemente, under the same climatic condi- 

 tions, should remain so magnificently perfect in their profiles. 



The inference from the absence of elevated strand lines is irre- 

 sistible. Santa Catalina has not been subjected to the uplift which 

 has affected the two prominent insular masses, one twenty-five miles 

 to the north of it and the other twenty-five miles to the south of it. 



More Ancient Stream Topography. — The stream topography of 

 the island testifies with equal force to the truth of this conclusion, 

 and the evidence which it offers is positive, direct, and unavoidable. 

 San Pedro Hill and San Clemente have both been shown to be ter- 

 raced plateau-like ridges, upon which stream action has as yet pro- 

 duced but little degrading effect. On Santa Catalina it is far other- 

 wise. The degradation of the island is well advanced, and the 

 original form of the mass upon which the streams first began to 

 operate, is no longer discernible. On San Pedro Hill and San 

 Clemente the stream trenches are comparatively few in number, and 

 are exceedingly sharp and precipitous narrow gorges, generally sep- 

 arated from one another by stretches of terrace flats. On Santa 

 Catalina the trenches are very numerous and are wide-open V-shaped 

 canons. The catchment areas of these canons are separated from 

 one another by sharp sloping ridges, formed by the intersection of 

 adjacent canon slopes. The entire island may be described as a 

 continuous alternation of canons and their separating ridges, there 

 being practically no plateau land remaining. None of the streams 

 are at base-level, but, on the contrary, have the high grade character 

 of mountain drainage. From this it will be evident that the topogra- 

 phy of the island is by no means ancient, as geologists usually apply 

 the word to stream topography. It is only the contrast with the 

 very much more youthful topography of San Pedro Hill and San 

 Clemente which justifies the use of the term. The relative age of 

 streams, near their headwaters in mountain ridges, is to be measured 

 by the mass of the mountain which they have removed, for, although 

 the streams be very ancient and endure through succeeding geolog- 

 ical periods, the topography at their head-waters will always have 

 the "youthful" character, till the mountain is obliterated. In this 

 sense the streams of Santa Catalina may be more ancient than many 



