34 P- T. CLEVE, 



II. III. IV. 



80,79 69,33 73,97 



11,13 12,77 12,09 



0,35 2,19 2,90 



0,21 .-. 7,23 — 



— 1,03 1,03 



4,22 4,75 3,38 



1,85 0,42 3,55 



1,26 1,47 1,54 





I. 



si 



63,89 . 



Äi 



15,08 . 



Fe 



4.63 



Ca 



9,00 



Mg 



1,06 



Na 



1,00 



K 



3,95 



H (ignition) 



1,45 



Sum. 



100,06 



99,81 99,19 98,46 



In the cretaceous formation felsite is a very important rock, and it occurs also in 

 a great number of different forms. Soraetimes the structure is compact, sometimes micro- 

 crystaline. In sorae varieties it is impossible to distinguish the component minerals, but 

 in others quartz in small spöts or pyramids may be seen distinctly, as at Cocolus Bay 

 and Grass Cay (S:t Thomas), in which case the rock is quartz porphyry, or else the feld- 

 spar is seen in small crystals in a compact mäss. In the latter case the rock may be 

 called felsite-porphyry. 



The rock occurs massive or in dikes and is consequently in all probability erup- 

 tive, but it occurs most often in stratified beds, and it is then an igneo-sedimen- 

 tary rock. 



The massive variety has sometimes an irregular angular cleavage, often its structure 

 is parallelopipedic (as in Virgin Gorda) or basaltic, (as near Red Point, S:t Thomas). 



Compact felsite interstratified with micaceous scales occurs in Dead-man's Chest. 



In the form of breccia the felsite is very common. The breccia exhibits a great 

 number of varieties, sometimes it has the coarseness of common sandstone, and sometimes 

 the pieces in the breccia are some inches in diameter. 



A peculiar variety of felsite occurs in the island of S:te Croix around the town of 

 Christianstaed. Here it forms a kind of variolite, very distinct when the surfaee of the 

 rock has been altered by atmospheric action. 



A special variety of felsitic breccia is the »bluebeache», the most common rock in the 

 Virgin-Islands. It is a breccia or conglomerate of felsite containing much hornblend com- 

 monly altered to a chlorite-like mineral, which gives the rock a dark-green and bluish 

 colour. Further on in treating of the breccias, I shall give a more complete description 

 of this rock. 



The colour of the felsite is commonly light, whitish, grayish, or reddish. Tlirough 

 alteration it commonly gets the rose or bloodred colour, which is so characteristic in this 

 rock on ihe southern coasts of S:t Thomas and S:t John. When the alteration is strong 

 the rock graduates into kaolin. Sometimes the felsite contains some mica or talc and may 

 then be called protogine felsite, and when the quantity of the latter ingredient increases 

 the rock graduates into talc-slate. In many places in the eastern parts of S:t John such 



