524 PROF. J. W. SPENCEE ON THE GEOLOGICA.L AND [NoV. I9OI, 



along the southern and south-eastern coasts. The numerous fault- 

 ings of the strata and the occurrence of dykes traversing the beds 

 are common to all alike, irrespective of their character. 



Beginning with Point Blanche, which is an isolated ridge from 

 500 to 600 feet above the sea, at the south-eastern extremity of 

 the island, and separated from the main range by a deep valley, 

 we find a thick formation of dark-grey siliceous limestones, mostly 

 lying in nearly horizontal beds, though faulted. Along the side of 

 the cove, at the eastern end of the ridge, the strata suddenly bend 

 down so as to dip 40° north-westward, while at the other end of 

 the ridge they locally dip and pass under the valley at nearly as 

 great an angle in a south-westerly direction. Many of the beds 

 occur in thin layers, but in some of the more massive the cal- 

 careous matter is intercalated with chert y laj-ers, often only 1 to 

 4 inches apart. In some cases the silicification is complete. The 

 whole series forms a very diirable mass, and gives rise to strong 

 topographic features. Some of the layers of these altered lime- 

 stones are composed of volcanic ashes or tufaceous sands. The same 

 rocks form the surface of the ridge, as well as the sea-cliffs, north 

 of Point Blanche. 



Along the southern coast, west of Philips burg, a similar 

 stratified formation constitutes the sea-cliffs. Generally its position 

 is nearly horizontal, but very much faulted. It rests upon hummocks 

 of the igneous basement-rock. The beds are mostly composed of 

 chert, with intercalations of volcanic sand or tuff. But the cherty 

 layers are often impure, from the admixture of the igneous debris. 

 The calcareous matter here seems to have been more extensively 

 replaced by the cliert than at Point Blanche. Near Pelican Point 

 some of the beds are rich in a manganese-oxide. Higher up the 

 hillside the manganese-mines of Governor D. C. Yan Romondt are 

 located.^ Here the strata are not separable from those nearer the 

 shore, except that the mineralization is more complete, which gives 

 them a different appearance. In the same layers the rich manganese- 

 accumulations are liable to be replaced by the chert of a jaspery 

 kind ; in other beds oxides of iron occur. 



It would thus seem that the mineralization and silicification are 

 due to the alteration of the original impure calcareous beds. But 

 even some of the interlaminated beds of volcanic debris are cemented 

 by silica, producing a complex rock. Thus the heavier jasper-beds 

 associated with the manganese -deposits east of the Philipsburg 

 salt-ponds have a kaolinized white surface, from the decay of the 

 felspathic impurities in the original limestone, or from the beds of 

 chert in some cases being altered tufaceous deposits. On the upper 

 Marigot Eoad the igneo-sedimentary rocks are well exposed to its 

 summit (350 feet above sea-level). These strata are consolidated 

 tuffs, penetrated by a dioritic dyke 1 foot wide. 



Although no fossils have been found in these deposits, yet the 

 conclusion arrived at places these beds, whether tufaceous, cal- 

 careous, or silicified, in one geological unit, consisting of a great 



^ The mineral is an impure psilomelanej containing a large percentage of 

 manganese. 



