Ijiiolixjij of Suvtltctisk-ni West India Islands. 187 



a compact mixture of quartz aud feldspar. In some places the 

 quartz is separated in the form of double pyramids. The rock 

 is evidentlVj in most places, a clastic rock, a kind of tufa; in 

 others, it seems to have been protruded in a molten state as a 

 lava. In the latter case, it has sometimes a fine columnar struc- 

 ture, as at Red Point on St. Thomas. 



3. Blue-heach. — This rock, so called by the inhabitants, is a 

 peculiar kind of breccia of fragments of felsitic or trappean 

 rocks, evidently a clastic volcanic rock. The color is generally 

 very dark green, from a considerable quantity of hornblende, 

 often altered to chlorite. In this rock, distinct traces of strati- 

 fication are often visible. The dip of the beds is generally very 

 steep, almost vertical. This rock constitutes the greater part 

 of St. Thomas, St. John, Tortola, and Jost Van Dyck. 



4, Diabase. — Occurs in dikes penetrating the diorite and the 

 blue-beach. Sometimes it occurs in greater masses, as in the 

 island of Hans Lollick, north of St. Thomas. The island of 

 Oulebra consists of a kind of diabase, or, more correctly, of 

 Labrador porphyry. 



All these igneous beds are of enormous thickness, and point 

 to a long period of very powerful volcanic activity. They are 

 of two different types, just as in modern volcanoes ; more basic, 

 black rocks, — traps, diorite and their tufas (or blue-beach), and 

 more acidic, — white or light colored, the felsites with their tu- 

 fas. The latter seem, also, older than the former. 



The Stkatified, gen^erallyMetamoephicEocks, have com- 

 paratively small extent ; many of them have doubtless been vol- 

 canic ashes, in a state of fine division, deposited on the bottom 

 of the sea. They consist of — 



1. Clay-slate. — K black slaty rock, without fossils, closely re- 

 sembling the slates of the Silurian formation. It occurs in the 

 northern part of St. Thomas, near Coki Point, and on south- 

 western Tortola, near Cox Head. The strata are almost verti- 

 cal, and run from west to east. 



2. MetamorpMc slates. — Mica schist, hornblende schist, etc., 

 occur on the small keys south of Francis Drake's Channel, and 

 on the islets between Tortola and St. Thomas. 



