1G8 



i)N THE TOPOGKArilY AXD GEOLOGY 



Yaqui. J^o outcrops occur on the plain, Init the l)luifs about Puerto Phita are made 

 up of the ordinary coast hmestone. 



Although where the road crosses the ridge which forms the base of Mount Isabella 

 the oldest rock found is the metamoi-phic sandstone just described, a little west of 

 the town where this l itlge reaches to the coast a very different condition of atiairs is 

 discovered. It is here found to be made up of the magnesian slate of the Sierra, 

 lithologically identical with the to pical localities in the central chain. Its most usual 

 character is the light greenish-gray semi-talc similar to that in the ridge between 

 Vega and Jarabacoa, or to that of many of the localities around the peak of the 

 Gallo. Its metamorphism is so complete that no stratification is discernible, though 

 the semi-lenticular flakes into which it usually l)reaks are more generally "on edge " 

 than horizontal in position, as if the dip was probably vei'tical or nearly so. As is 

 intimated in the preceding paragraph, the presence of this intrusion of Cretaceous 

 nnder the nearly horizontal Miocene is sufficient to account for the alteration of the 

 latter, when we bear in mind that similar causes have produced like effects at 

 Tabera, and that in going east from the locality in question on the Palo Quemado 

 route tlie metamoi'phism gradually dies out. Mount Isabella de Torres, the best land- 

 mark on the north coast, after losing sight of the headlands of Samana on one side 

 or Monte Cristi on the other, is a flat mountain which gave to Mr. Pennell a height 

 of 2,530 feet. Schomlmrgk mai'ks it in round numbers at 700 metres. One or the 

 other is evidently in error. The latter, in his account of his visit to Constanza, in the 

 AthencEmn Journal, speaks of his having used on thjit trip an aneroid barometer, and 

 it is not improbable that in this case his measurement may have been made with an 

 equally unreliable instrument. Mr. Pennell's observations Avere made with one of 

 Green's best mountain barometers of the Smithsonian pattern, an instrument whicli 

 the extensive mountain work of the Californian Geological Survey has proven to be 

 without a superior. By comparison with my office standard it was proven to be in 

 perfect condition l)oth befoi'e and after the observation, so I can liardly imagine the 

 possibility of an eri'oi'. There is certainly none in the computation of his observations. 

 This mountain is capped with a thick bed of white limestone, below the edge of wdiieh 

 the talus covei's up all outcrops, so tliat the details of its striictin-e are not accessible; 

 but we are bound to infer that the highest member of the formation being found at 

 its summit and the oldest being seen at the level of its base, the intermediate strata 

 ni-e in all probability rej)resented at their various levels in its interior. W e have thus 

 a pretty good criterion for judging of the thickness of the formation at this point, as 

 stated in the preliminary observations on the range. 



West of Puerto Plata a narrow strip of coast limestone borders the sea, making 



