GENERAL CONCLUSIONS. 513 



The little that is known of the structure of the underlying or pre- 

 Cretaceous mountain system of the peninsula seems to indicate that its 

 larger general features have an en echelon character that is so common 

 among the larger chains of the Cordilleran S3^steni of which the Colorado 

 Front range of the Rocky mountains is a most striking example. In the 

 present case the chain taken as a whole has a north-northwest trend, 

 while its component mountain ridges have a trend of about northwest, 

 with considerable depressions or bays opening out to the nortliwest be- 

 tween the overlapping ends of these ridges. 



In the gradual submergence which followed the period of folding and 

 erosion that culminated, according to the latest evidence, about the close 

 of the Jurassic time, the first Cretaceous beds would have been deposited 

 in the bottoms of these longitudinal depressions, and if they grew shal- 

 lower or were closed to the southeast by the saddles between tlie echelon 

 ridges along the main divide, the earlier beds would have been confined 

 to tlie region west of the divide, even if the larger area to the east of the 

 divide had alread}^ commenced to sink. If the continental submergence 

 continued, as it evidently did in Lower California up to near the close 

 of the Tertiary, the later deposits might have extended entirely across 

 the peninsula without showing any evidence of discordance or want of 

 continuity with the earlier l)eds. 



An examination of the shore platform or limit of the 100 to 200 fathom 

 curve around the coast of Lower ('alifornia, which is generally regarded 

 as the proper dividing line between the continental and oceanic areas, 

 shows that the peninsula is a very narrow mountain mass, descending 

 generally to the southward and i)rol)ably dying out entirely in the Pacific 

 ocean to the west of the coast of Mexico. On the east the gulf of Cali- 

 fornia, north of the straits formed by the great islands of Tiburon and 

 Angel de la Guardia, belongs to the shore platform or forms i)art of the 

 valleys to the- north, having no depths Ijelow 200 fathoms, while south of 

 these straits its bottom drops rapidly to o,000 feet. On the west coast 

 the lOO-fathom line follows a comparatively direct course from headland 

 to headland, disregarding the great reentering curves like Viscaino bay, 

 while oceanic depths of 1,000 to 1,500 fathoms are found within 20 to 

 30 miles seaward from these headlands. Between the headlands the 

 l)resent coastline shows a tendency to form loops opening out to the 

 northwest, which suggest the en echelon dejn-essions in the pre-Cretaceous 

 toi)Ography alluded to al)ove, and which would bear a closer resemblance 

 to their form had not recent eruptions of volcanic rocks interfered with 

 the uniform degradation of the soft beds which were deposited in them. 



The first dei)ression south of the international boundary is the very 

 small one at Todos Santos bay, in which but a small remnant of Chico 

 beds remain. From there to cape Colnett the older rocks form the im- 



LXXI-Boi.i.. GKor,. Soc. Am., Vol. 5. 1893. 



