1V2 



ON THE TOrOGKAPHY AND GEOLOGY 



PART III. 

 LOCAL GEOLOGY. 



^ CHAPTER VIII. 



'x. 



GEOLOGY OP THE CENTRAL MOUNTAIN CHAIN. 



It is hardly necessary to say anything more about the physical features of the 

 great central chain. A reference to the first chapter, aided by the map, will give the 

 reader all the details necessary to a clear understanding of the following description 

 of its geology. Covering a length of at least 200 miles, with an average width of 

 not less than 30 miles, its area may be roughly estimated at a little over 6,000 scpiare 

 miles, or practically a third of the Republic. Including the spurs running down 

 towards the coast near Bani, the actual area is really greater than the above estimate, 

 which is near enough, however, for all practical purposes. 



The chain is composed of a central core of eruptive rocks, which have upheaved 

 and contorted the sedimentary strata overlying and flanking it. This core does not 

 extend the entire length of the chain, but begins near the middle as a great, irregu- 

 larly-shaped mass lying oblicpiely across the axis of the range. Its eastern end 

 is abruptly cut ofi' in the region of the Jaina River, but to the Avest it stretches out 

 in a series of parallel ribbons at least to, and perhaps beyond, the borders of the 

 Republic. Overlying this rock, pushed up, folded and broken by it, is the great 

 series of slates, conglomerates and limestones belonging to the Cretaceous formation. 

 These cover the greater part of the area, not only of the mountain mass, but of 

 the Island, and are the oldest formation yet discovered within its limits. 



The foldings of the Cretaceous beds and the lines of exposure of the eruptive rocks 

 correspond almost exactly with the trend of the chain. This is not exactly the same 

 throughout, although the amount of variation from a straight line is very little, and 

 the average direction is about 70° W. It is worthy of remark that the eastern 

 half of the range, or that part east of the intrusive rocks, is nearly due east and west, 

 while the remaining portion turns up cpiite abruptly to the northwest from Mount 

 Vanilejo. This eastern half is much less disturbed ; the foldings are not so compli- 

 cated nor numerous, the absolute elevation is much less, and the upheaving force was 

 evidently expended, in great part, towards the centre of the Island ; the portions 



