536 PROF, J. W. SPENCER ON THE GEOLOGICAL AND [NoV. I9OI, 



III. The Volcanic Formations. 



The remnants of the old igneous foundation are seen in the 

 denuded hills and valleys of the ' Salt Ponds ' district, or the south- 

 eastern extension of St. Kitts. The topographic features resemble 

 those of St. Martin and the mountain-district of Antigua. Similar 

 features are seen in the northern part of Statia. Everywhere else 

 these beds are covered by the much more recent volcanic forma- 

 tions. The newer materials are mostly incoherent volcanic ashes 

 or cinders, much decomposed, with small angular pebbles and 

 occasional boulders of trachytic character, containing crystals of 

 triclinic felspar, liable to become white from kaolinization, and 

 some hornblende. At some of the higher altitudes the beds of 

 tuff are more coherent. At all the lower elevations the surface of 

 the island is covered by re-washes of loose material. The stratifi- 

 cation of the deposit dips outward. At one place only were lavas 

 seen, namely, at a point on the northern shore, 11 miles from Basse 

 Terre, where a sheet of black basalt occurs at a height of 25 feet 

 above the sea, covered with marine sandy tuff showing signs of wave- 

 action. Dr. Christian Branch states that no other lava-deposits are 

 exposed on the island, except perhaps some ' black rock ' reported at 

 points difficult of access among the mountains. The character of the 

 igneous rocks has not been made a subject of special study by me, 

 as the volcanic phenomena were not the primary object of my visit 

 to the islands ; but their relationship to the other formations is an 

 important question. No eruption is recorded as having occurred 

 during the historic period upon any of these islands, although they 

 are still visited by severe earthquake-shocks. The ridges, which 

 are being rapidly dissected by the enormous precipitations of 

 moisture from the trade- winds, are still more or less intact. The 

 tuffs also extend under the marine deposits forming the present 

 floor of the sea, as shown at Brimstone Hill, where these have 

 been thrust upward by a local volcanic upthrust, which is inde- 

 pendent of the general changes of level that have affected this area. 



IV. The Limestones of Brimstone Hill. 



Brimstone Hill is a secondary and adventitious volcanic dome, 

 having a diameter at the base of about half a mile, and its summit 

 reaches a height of 700 feet. It is composed of loose or semi- 

 coherent tuffs, the beds showing intense contortion and fracture. It 

 IS covered by a mantle of white marl or limestone, from 15 to 30 

 feet thick, much fractured and dipping everywhere outward from 

 the central dome, even at angles approaching the vertical. The 

 mantle occurs only to a height of 450 feet. This feature is repeated 

 at the southern end of Statia, 12 miles away, where the limestone- 

 mantle has been carried up to a height of 900 feet, upon the flanks 

 of the crater-cone, which rises to an altitude of 1950 feet, and is 

 still well preserved, although composed only of cinders. But the 

 volcanic activity which produced Brimstone Hill does not appear to 



