1853.] HENEKEN — SAN DOMINGO. 121 



but the tufaceous limestone, which has aiforded no satisfactory lines 

 of stratification, is presumed to be horizontal. 



These beds always maintain the same relative position and con- 

 tain the same group of fossils ; and the Grange Mountain, the only 

 outlier of shale on the north bank of the River Yaqui, exhibits 

 a similar arrangement. The body of the mountain is composed of 

 the blue shale covered by the tufaceous limestone in a tabular mass, 

 and these are separated by the intercalated bed of shingle. At the 

 western extremity of the Grange is the cliff above alluded to, that 

 presents so interesting a section of the interlacing of the yellow and 

 the blue sedimentary deposits (fig. 5). Upon the south face of the 

 mountain the line of the two shales may be distinctly traced by 

 the difference of the prevailing vegetation. The blue shale favours 

 the growth of a kind of coppice, while the yellow shale is thinly 

 covered with tufts of coarse grass. 



I have tried to explain to myself how these two different kinds of 

 sediment could have been deposited contemporaneously, and I ven- 

 ture to suggest the following explanation. The valley of the Yuna, 

 to the eastward of Santiago, is, geographically speaking, a continuation 

 of the valley of the Yaqui, and also contains tertiary deposits. At 

 the time of the formation of the tertiary beds of the Yaqui Valley, 

 these two valleys were in all probability one continuous sea or narrow 

 strait, contracted about the middle. The same Equatorial current 

 from the eastward that now prevails must then have existed, which, 

 setting through the narrow strait, gradually silted it up with detritus 

 from the surrounding mountains, while a current (possibly a river- 

 course) from the southern side of the head of the basin furnished 

 contemporaneously the yellow calcareous sediment. 



As the shales gradually attained the surface, they were covered by 

 shingle transported from the central chain of mountains ; and sub- 

 mersion taking place, it is presum^ed that the same source whence 

 the former calcareous sediment proceeded, being somewhat modified, 

 then continued to supply the tufaceous limestone deposit ; and which, 

 I would suggest, might have been derived from a vast coralline forma- 

 tion to the south-east, which will be reported upon at a future day. 



The blue shales may be distinctly traced in contact with the 

 chloritic schists and secondary rocks along the south-eastern limits 

 of the basin (see figs. 2, 3, 4) ; but they gradually thin off along 

 their northern edge from Santiago to the westward from the effects 

 of denudation, and they now appear as a narrow band, partially 

 capped by the tufaceous limestone, which forms the chain of hills, 

 called the Samba, intermediate between the two mountain ranges 

 of Monte Christi and the Cibao. 



That these tertiaries formerly covered the whole plain is very 

 apparent on comparing the structure and formation of the Grange 

 with the hills of Samba. Additional testimony of the extent which 

 these tertiaries have occupied exists in the abvmdance of tertiary 

 fossils strewed over the surface of the sandstone plains, and found 

 imbedded in silt apparently derived from the destruction of these strata. 



The formation of these beds seems to have been affected by pro- 

 cesses even now in action, for in the environs of Santiago, or at least 



