118 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Mar. 23, 



lated appearance. The thickness of this sandstone cannot be less 

 than 500 feet, and it is underlaid by a large development of conglo- 

 merates and a coarse gritstone, that crop out at the foot of the 

 Monte Christi range of mountains, but no fossils have as yet been 

 detected in them. 



The River Yaqui takes its rise in the Cibao Mountains, and, pur- 

 suing a southerly course, meets the Tertiary beds near Santiago, which 

 are here cut through, leaving on either hand perpendicular cliffs 

 nearly 200 feet high ; it then turns suddenly to the westward, and 

 continues its way through the entire length of the sandstone plain, 

 fringed on both its banks by a rich alluvial soil, averaging above two 

 miles in width. 



The height of the Monte Christi range is estimated at 3500 feet in 

 the vicinity of Santiago, and its southern flanks are chiefly composed 

 of a compact limestone affording good marble. This cordillera gra- 

 dually diminishes in height as it approaches Monte Christi, where it 

 terminates in low hills of shale and sandstone. 



Coal (lignite) appears in the Monte Christi Hills at about long. 

 70° 32' and long. 71° 7', W. of London, and at other places along 

 the range. 



At some distance from the western extremity of this cordillera, and 

 at the northern point of Monte Christi Bay, is the Grange, a steep, 

 isolated table-mountain, above 900 feet high, in the same line as the 

 cordillera, though of a newer formation. This mountain, as an inter- 

 esting outlier, I shall revert to hereafter. 



The Cibao Mountains, on the south of the River Yaqui, are 

 chiefly made up of granite, greenstone, porphyry, quartz, chloritic 

 schists, and other crystalline rocks. 



Secondary (?) and metamorphic rocks skirt the Cibao Range at Las 

 Matas and to the south-east ; and again from between the Samba 

 Hills and Savaneta to the westward. On this side of the Yaqui Val- 

 ley coal occurs on the River Yaniq\ie, and about 7° W.N.W. of 

 Savaneta. 



Upon the extensive sheet of sandstone that fills the interval be- 

 tween the two ranges of mountains just described, the Tertiary beds 

 of Santiago have been deposited (see Map and Sections, figs. 1, 2, 3, 4). 

 Subsequently, a very considerable portion of these deposits has evi- 

 dently been stript off by denudation, particularly from the northern 

 and western parts of the valley. At the eastern extremity or appa- 

 rent head of the basin, near the town of Santiago, they are tolerably 

 perfect, and fill up the plain from one range of mountains to the 

 other. (See fig. 4.) 



The principal member is a bluish or greenish sandy shale, more or 

 less affected by the colour of the neighbouring rocks from which it 

 appears to have been derived. Occupying a central position along 

 this blue shale, and following more or less the direction of the Samba 

 Hills, is a yellow calcareous shaly deposit of the same age as the 

 shale, and in some places covering, and in others interlacing itself 

 with it ; but evidently derived from a distinct source. This yellow 

 deposit has been satisfactorily traced from the head of the basin to 



