JuNS 21, 1907] 



SCIENCE 



977 



3,000 and 5,000-foot contour line. The upper 

 margin of the slopes of the mountains of the 

 southern part of the Great Basin has an 

 average height of 3,466 feet, the mean of the 

 height at eight different localities ranging 

 from 3,000 to 4,000 feet. 



The Santa Catalina range of mountains 

 north of Tucson affords an excellent example, 

 especially along the south side, where, for a 

 distance of nearly fifteen miles, the slope com- 

 mences at the base of the cliffs at about the 

 contour line of 3,000 feet. The change of the 

 topography may be followed by the eye. It 

 has the semblance by its horizontality of a 

 beach line or old shore line, but the usual 

 characteristics of an old beach or ocean 

 border, such as shingle and upright sea- 

 wrought cliffs, are wanting, having probably 

 been obliterated by long-continued weathering. 

 The same contour line of '3,500 feet may be 

 followed similarly around the western point of 

 the range to the north side. In other words, 

 the 3,500-foot contour line follows approxi- 

 mately the beginning of the rocky outcrops 

 above and of the slopes below. From this ele- 

 vation up to 4,000 feet appears to have been 

 the height of the old ocean level for a long 

 time. It can not yet be fixed with precision. 

 Careful search along this general level may 

 result in finding unequivocal evidence of the 

 former level of the sea. In the Huachuca 

 Mountains the upper margin of the slopes is 

 higher, being nearer the 5,000-foot contour 

 than the 3,000-foot. 



We do not find along the Catalina range, 

 a distinctly scalloped outline of intersection 

 of the slope with the higher ridges. The 

 slope is very even and regular in its con- 

 tinuity and in the approximate parallelism of 

 the contour lines, broken only by the arroyos 

 or channels of modem drainage, leaving 

 remnants of the former slope on each side. 

 The same observation applies to the slopes 

 bordering the Santa Rita and other ranges 

 north and south of Tucson. These conditions 

 are well illustrated by the contour lines on 

 the map by the U. S. Geological Survey of the 

 Tucson and Santa Eita quadrangles to which 

 reference is made. 



Slopes Older than Existing Drainage.— That 



these ancient detrital slopes are older than 

 the present streams and watercourses is made 

 more evident and in a striking manner by the 

 fact that existing streams and valleys cut 

 across the slopes at right angles to their line 

 of descent, in some instances bisecting the 

 elopes as on the San Pedro River, between its 

 junction with the Gila river and the town of 

 Mammoth. One part of the long slope from 

 the Catalinas is found on one side of the 

 stream, and another portion on the opposite 

 side. 



The slope of the southern side of the Santa 

 Catalina, a few miles north of Tucson, is 

 cut across for miles at right angles to its 

 southward descent by the east and west stream, 

 known as the Rillito, which has left an 

 abrupt bank on its north or right bank, a 

 truncation of the slope, while on the south 

 side or left bank we find upon the ' mesa ' the 

 coarse detrital deposits characteristic of the 

 ' wash ' or sloi)es of the Catalinas. It is clear 

 that these deposits were once continuous and 

 that they were laid down before the Rillito 

 valley was cut out. Semi-rounded blocks of 

 large size, a foot or more in diameter, of the 

 peculiar augen-gneiss of the Catalina ridges, 

 and horizontal beds of gravel and sand from 

 the mountains are found in digging the welli 

 of the mesa south of the Rillito. Obviously 

 this distinctly detrital slope was formerly un- 

 broken in its extension to the Santa Cruz, and 

 was formed before the Rillito valley, which 

 has been cut out of the slope by the river. 



Similar conditions are found farther south 

 where the Santa Cruz River cuts across the 

 lower portion of the slope from the Santa 

 Rita mountains, north and south of Tubac. 

 The cutting out of this valley leaves a steep 

 bluff-like ending of the long regular slope. 

 This also has many deep arroyos parallel to 

 the line of descent, which show the rude 

 stratification of the detrital materials of the 

 slope, portions of which are soft and clay- 

 like, apparently deposited under water. In 

 some sections, the stratification is very regu- 

 lar and horizontal. The portions of the an- 

 cient slope left between the arroyos have 

 practically the same angle of descent and the 



