AND PORTO SANTO. 87 



summit of the table land, of which this island is principally 

 formed, to the sea, a depth of about 240 feet) descend, more or less 

 perpendicularly, numerous basaltic dikes, sometimes jutting out 

 like walls, and serving as rude stairs in the ascent, at others, nearly 

 even with the surface of the various rocks they intersect, and fre- 

 quently running parallel with the beach for some distance, at the 

 water's edge, and forming rude piers. In some parts, their surface 

 was covered with considerable patches of a dull coralloidal carbonate 

 of lime, and in the basalt of the dikes on the north side, (for it was 

 the eastward face which afforded me the section I have described) 

 I found beautiful crystals of nepheline. A composita (jiosculoms) 

 with white decomposed leaves, a ligneous stem, and flowers borne 

 in large close panicles, characterized the whole of this limestone 

 island ; the absence of the florets prevented me from determining 

 it ; the involucra were polyphyllous, and the receptacles covered 

 with silky hairs. Several small masses of the spongia officinalis lay 

 on the beach. 



The peaks of tufa in the north-eastern part of Porto Santo, are 

 capped with a basalt approaching to phonolite, (Avhich, if I mistake 

 not, has been found to cap the basaltic mountains of Bohemia and 

 other parts of Europe) by its hghter colour, numerous vitreous 

 crystals of felspar, decomposing crystals of common hornblende, 

 and lesser specific gravity, 2.15; but it yielded no particular 

 sound hke phonoHte, and its lamellar structure deviated in one 

 instance into large pentagonal columns ; immediately beneath this 

 capping of hghter basalt, appear the dikes, which descend through 

 the tufa to the sea ; the upper parts of these dikes (sometimes 

 elevated 1600 feet above the sea), are generally of an earthy brown, 

 ferruginous appearance, but as they approach the sea, the basalt 

 becomes of a dark grey colour, and in the north-eastern point of 

 the island, especially, near Pico da Cruz, it is studded with large, 

 but imperfect, crystals of basaltic hornblende. In descending by 



