172 



ON THE TOrOGRAPIIY AND GEOLOGY 



actly along the strike of the rocks, and fiii-ther, except crossing a very low ridge 

 almost its whole length is in the bed of a creek, which it crosses, stumbling among 

 sandstone boulders so many times that one gives u]) counting in despair. 



The rocks on the Jobo trail while striking as usual about east and west, show a 

 more than ordinaiy amount of disturbance. ISTear the mouth of the river, heavy 

 bedded sandstones and conglomerates are found which continue to and beyond the 

 little settlement of two or three houses called Blanco. The resemblance of some of 

 these coarse-grained sandstones to those near Puerto Plata, which are more or less 

 metamorphosed, is so striking that I could not resist the impression that they are the 

 same beds, ^Nowhere else, beyond the north face of this range, unless it be on the 

 south edge of the formation on the Bao and Yaqui, have I observed the peculiar 

 appearance possessed by them. It is one of those intangible characteristics that one 

 recoofnizes but cannot describe. It is nothino- remarkable ; nothino- more than a 

 similarity in "grain" and in general appearance. The dip changes constantl 3^ ; some- 

 times it is north, sometimes south, and its angles are also sometimes quite high. At 

 Blanco, which is but a short distance north of the summit, there are thick beds of 

 sandstone studded full of large pebbles which dip about 25° south. Above this 

 point, the more recent members of the formation appear regularly with gradually de- 

 creasing southern dips until at the summit we find the white earthy limestone capping 

 the range and neai'ly horizontal. Further south, descending towards Macoris, the brown 

 shale is again met under the limestone dipping south and passing under the valley. 



About five miles south of Blanco, on the trail, is a dyke of syenite about 200 yards 

 wide, cutting directly across the I'oad. The exposures at its sides were so small and 

 so covered up by the soil that I was not able to ascertain whether its presence pro- 

 duced a local metamorphism of the adjacent shale. Like the boulders in the Yasica, 

 it bears a remarkable resemblance to the intrusive rocks of the Cibao I'ange. Where 

 I crossed it, it was coarse-grained and composed of white feldspar, but little quartz 

 with black hornblende, and contained a green mineral resembling augite. A mile or 

 two east it is cut by a stream running down to Macoris, from the bed of which I col- 

 lected specimens much finer-grained and without the green mineral. If these speci- 

 mens were mixed with the Sierra series they would defy the most expert petrologist 

 to find a distinguishing character. And yet the Sierra slates were upheaved by these 

 intrusive rocks, and the Tertiary deposited indiscriminately over them and over the 

 upheaved edges of the slates. If further proof of the pre-Tertiary, or rather pre- 

 Miocene age of the Sierra syenites is required, the presence of pebbles of these rocks 

 in the conglomerates at or near the base of that formation furnishes it. And here 

 we have similar syenites cutting through these very beds of conglomerate. 



