156 



VN THE T(JPOGKArHY A^T> (iEOl.OGY 



Before proceeding to a description of the north .side of the yalley, it is probably 

 advisable to investigate the relation of the gravel beds and their accompanying sand- 

 stone and clay strata to the limestones and white marls which cap the Monte Cristi 

 range, and which Ibrm the southern border of the formation near Samana Bay. As 

 has been demonstrated, 1st, wherever the gravels occur they invariably constitute 

 the top of the series ; 2d, when in contact with older members of the same forma- 

 tion they always overlie brownish or yellowish shales which generally carry beds of 

 lignite, and in turn rest upon darker-colored (usually blue or bluish) shales ; 3d, they 

 ai'e never found in the same area with the limestones ; and finally, they are a shore 

 deposit, the origin of which can still be distinctly recognized in the Mao and lesser 

 neighboring mines, and in the contained pebbles and even gold, Avhich alike point to 

 the present central Cordillera, then simply a smaller island, as their source. The 

 whole area covered by the deposit is a long narrow triangle, its base opening like a 

 fan to the westward and its apex between the Mao and the Amina Rivers, perhaps 

 not more than fifty miles long and averaging ten miles wide, unless we include the 

 sandstone about Monte Cristi, which widens it at that point to nearer twenty. About 

 the Mao the pebbles are often angular and boulders of great size are very common ; 

 but as the distance from this point increases, the large boulders become more rare, 

 and there is a marked diminution in the average size of the pebbles, facts which 

 clearly point to the Mao as the great source of supply, and to a current from east to 

 west as the means of distribution. Its southern edge is everywhei-e in contact with 

 and overlies horizontally the upturned edges of the Sierra slates, while its northern 

 margin either thins out, as near Cercado on the lower Mao, or changes to a finer sand 

 without pebbles, as on the Guayubin River. , 



On the other hand, the limestones (in which term I desii'e to include the true 

 limestone-like parts of the north face of the Samba Hills, the rocks on the summit of 

 the Monte Cristi range on the Alta Mira and Isabella Passes, those of Samana and 

 south of Savana-la-Mar, the calcareous sandstones of San Lorenzo, and the white 

 calcareous marls north of Moca) will be found to be equally amenable to the above 

 laws, first and second ; by changing the titles of the rocks the third rule applies, of 

 course ; and although about Samana Bay they are really shore deposits, these deposits 

 were made in clear water, Avhere no river brought down sediment or diluted the salt- 

 ness of the sea. It is a fact not less durions than interesting, that in the same area 

 there should be two formations whose whole history should be so perfectly preserved 

 and of which one should be so perfectly a repetition of the other, as is the case 

 between the gravel and the lime-beds of the Dominican Miocene and Post Pliocene. 



