ON THE GEOLOGY OF THE NORTH-EASTERN WEST-INDIA ISLANDS. 5 



side of the island there are almost vertical stratä of black clay-slate. One can trace the 

 stratum to the North-side Bay, where it is covered by a loose clay, containing shells of 

 living species. It continues towards the east to Mandal and is seen near Coki Point. The 

 clay-slate bears a perfect resernblance to silurian slate, but T have not been able to find 

 any trace of fossils in it. In some places the rock is associated with liinestone nodules, 

 containing pieces of bluebeache-conglomerate. In the coast-clitfs by Tutu-Bay, north of 

 the clay-slate band, is to be seen a regularly stratified rock dipping 70° north and stri- 

 king east to west. In the cliff west of Tutu-Bay the strata are almost vertical, and in 

 the point between Tutu-Bay and Mandal-Bay the dip is southerly. The rock is composed 

 of smaller granules or pieces of a gray compact felsite, bedded together to a sandstone- 

 like flagstone. When the rock is fme-grained it resembles clay-slate of a light colour. In 

 the large branch north of Bucks Bay occur bluebeache, black and sometimes metamor- 

 phosed clay-slate and flagstones alternating. Near Coki Point the bluebeache conglemerate 

 appears again, but here it contains large masses of calcarious nodules and marble of a 

 white or gray colour. The limestone occasionally contains fossils, mostly in a very bad 

 state and sometimes silicified. The most cornmon are Nerincece and fragments of a large 

 Bulla or Acteonella. Beside those fossils I have found an Ammonite, Trochus, Pectunculus, 

 Limopsis, Opis, Venus, Altarte, Corbula etc. Some of thein have a remarkable affinity to 

 cretaceous species, and I have no doubt that the rocks of S:t Thomas and the other Virgin 

 Islands are of the cretaceous age. 



The Favosites Dietzii and S:t Thomas Mich. Duchas*) are said to have been found 

 on the spöt, but I have not found any fossil of paleozoic appearance near Coki Point. 

 The Favosites Dietzii, of which I recieved one silicified specimen from Mr Dietz of S:t 

 Thomas, is scarcely different from the silurian F. Forbesi, and, when I dissolved the spe- 

 cimen in hydrochloric acid, I found some other silicified fossils, fragments of small bra- 

 chiopode-shells and encrinite-stalks etc. There is no doubt that the Favosites Dietzii is 

 of paleozoic origin, and I can scarcely believe that this coral is found in the West- 

 Indies. 



Following the coast from Coki Point to Red Hook one meets with another kind of 

 rocks, often distinguished at a distance by its red colour. It occurs in Cabrite Point 

 and along the southern shore of S:t Thomas, by Long Bay and Cocidus Bay to the en- 

 trance of the harbour. It continues in Water Island, Regis Point, Mosquito Bay Point and 

 Red Point, but does not extend west of the latter spöt. The chief component of this rock 

 is felsite or a mixture of quartz and feldspar. In different places it has a variable appea- 

 rance. At Red Point it is a breccia or conglomerate, sometimes so fine-grained, that the 

 rock resembles chalk or domite. At Mosquito-Bay Point and at Regis Point it is a härd, 

 compact rock of a beautiful basaltic structure. Near Cocidus Bay and on the small Grass 

 Cay the rock contains quartz in pyramids and is a kind of quartz-porphyry. In some 

 other places, smaller feldspar-crystals occur in a homogeneous mäss, and the rock is then 

 a kind of felsite-porphyry. The rock produces by alteration a kind of white clay or kaolin 

 often penetrated by veins or fissures filled with red or brownish-red hydrate of iron and 



') Mera della Acad. dell. Scien. di Toriuo II Ser T. XIX pag. 84. 1860 and. T. XXIII (1866) pag. 199. 



