4 P. T. CLEVE 



westward by Puerto Rico and the greater Antilles. The geology of those islands is very 

 little known. A general account of them was given in 1837 by Rob. Schomburgk*) and 

 S:t Thomas was described by Dr Hornbeck**) in 1840. 



1. S:t Thomas, situated at 18° 20' 40" N. Lat. and 64° 55' 38" Long. fr. Greenw. (at 

 the town of Charlotte Amalia) is narrow and its greatest lengtli from east to west is 

 about 22 kilometres. From north to south S:t Thomas is scarcely more than from 3 to 

 6 kilometres in breadth- The middle part of the island forms a tolerably high ridge, with 

 summits of 470 met. above the sea-level. The ridge is highest north of John Bruce Bay 

 and slopes gradually to the west and east, branching off in two low series of hills. The 

 north side dips down more suddenly in the sea, than the more hilly southern part. Fresh 

 water is very scarce, most of the rivulets being periodically dried up; the only permanent 

 spring is near Lilljendal. Along the coasts, especially on the south side, are several shallow 

 lagoons with brackish water. The nature of the rock, of which the greatest part of the 

 island consists, inay easily be studied in the harbour of Charlotte Amalia. It is a con- 

 glomerate, called by the inhabitants »Bluebeache», and is sometimes found distinctly stra- 

 tified. Its colour is a dark bluish-green. This conglomerate or breccia consists of angular 

 pieces of dark porphyry or felsite and also rounded scoriacious stones with the cavities 

 commonly filled by quartz or calcarious spar. The cementing part of the stone has a 

 dark greenish colour, probably derived from decoinposed hornblencl. In the liarbour near 

 the fort and on the small cay of Prins Rupert the rock forms regular strata of half a metre 

 in thickness. Their strike it S. S. E. — N. N. W. and dip 30° to the E. N. E. The rock has 

 evidently been exposed to a strong metamorphic action so that the stratification in most 

 other places can scarcely be seen. In the town and near Fredriksberg the rock is very 

 härd and solid, often penetrated by veins containing white quartz, calcarious spar and some 

 epidote or pyrites. On the high ridge in the middle of S:t Thomas several interesting 

 varieties occur, resembling diorite or syenite. Where the rock is more fine-grained it has 

 a great resemblance to trap. The latter variety occurs in the hills north of Mosquito-Bay, 

 where it has a beautiful concentric spheroidal structure, visible when the surface has been 

 exposed to atmospherical action. Some of the globules are \ — 1 metre in diameter. 

 Through alteration the rock is sometimes decoinposed into a white clay, called marl, as 

 near Altona. The rock is often penetrated by dikes of felsite or diabase. The ocurrence 

 of scoriacious pieces in the conglomerate indicates its volcanic origin and the stratification, 

 as well as some large limestone-nodules with fossils found in one spöt, proves that the 

 material is heaped under the sea. Near John Bruce Bay the conglomerate has a red or 

 brownish colour and contains many smaller scoriacious pieces. The hills and mountains 

 in the western part of the island consist of bluebeache and the same rock also occurs 

 east of the town often associated with large masses of felsite, as near Donoe. The blue- 

 beache seems to me to be at least 2,000 metres thick. Near Löwenlund on the north 



*) Die Jungfrau Inseln in Geologische und Klimatologische Hinsicht in Beeghaus Alraanach fur Erdkunde 

 1837 p. 366. 



**) Nogle bemserkrringer över S:t Thomas' geognosie. Skandinav. Naturforskere andet Mode 1840 p. 364. 

 (Kiöbenhavn 1841). 



