The Tertiary of Soiitliern Lower Californiia. 548 



Comondu and Loreto, are composed of heavy masses of basaltic 

 breccia. Unquestionably, the conglomerates and sandstones have 

 been hroiiglit from east and north-east, where now lies the Gulf of 

 California. This idea was first suggested by Gabb. 



The thickness of the Comondu Formation increases rapidly toward 

 the interior of the Peninsula. At the Syncline Head of La Purisima 

 (PI. XXII) it was measured 65 metres, at the Pilon of La Purisima 

 150 metres (Fig. 3), south of Ojo del Agua (eye of water or spring) of 

 the Arroyo Cadegomo it was esteemed to about 300 metres, and in 

 the region of La Giganta Mountain it is even more important. The 

 total thickness of the unfolded beds of La Giganta Mountain, 

 including the basalt breccia, seems to be more than 1,300 metres. 



No fossils have yet been found in the Comondu Formation. It 

 seems to be a continental deposit of great extension. Indeed, it 

 was observed by the writer along the Cadegomo River from below 

 La Purisima (Syncline Head) to Ojo del Agua, and from Comondu 

 to La Giganta Mountain for more than 50 kilometres across the 

 Peninsula. Also it occurs in San Bisente Canyon, between Purisima 

 and Comondu. As seen through the telescope, it seemed that the 

 high mesas north-east of the Oasis of San Hilario also are formed 

 by unfolded Comondu Formation. Finally, the coarse sandstone 

 of Los Martires Hill at the Gulf side of the Cape Range, lat. 23° 36', 

 already mentioned as possibly Miocene by Gabb, and other outcrops 

 along that part of the coast, have reminded the author of the 

 Comondu Formation. 



Although generally horizontal, the Comondu Formation has been 

 gently folded in some places. The most beautiful fold is the syncline 

 of Syncline Head (PI. XXII), where the Comondu Formation, in 

 travelling from the Pacific coast, appears for the first time in the 

 Cadegomo Valley. Another locality where the Comondu beds are 

 inclined is the rock called El Devisadero, to the north-east of 

 San Isidro. 



The age of the Comondu Formation is regarded as probably Upper 

 Miocene, possibly even Lower Pliocene. 



CuESTA Formation. 



At La Cuesta de la Purisima, a distance of TT kilometres south- 

 south-west of the mission, the Comondu Formation, at an elevation 

 of about 100 metres above the river, is overlain with a slight uncon- 

 formity by sandstones for which the name Cuesta Formation is 

 proposed. The lower part of the sandstones is grey, while the upper 

 looks more brownish and resembles the Comondu Formation. 

 They reach the " Cuesta " ( = the rim of the mesa), which is at an 

 elevation of about 270 metres above the Cadegomo River. 



A similar observation was made at the Syncline Head (PL XXII), 

 where the brown Comondu sandstone is overlain with a slight 

 unconformity by the horizontally stratified grey beds of the Cuesta 

 Formation. The thickness of the latter, at this place, is about 

 11 metres only. 



