14 GEOLOGICAL MEMOIRS. 



basaltic greenstone is in the shape of dykes of various thickness, but 

 so numerous, that it forms nearly half of the whole mass. These 

 dykes appear at the surface towards the edges of the formation, but 

 in the middle are covered with compact beds with a smooth exterior 

 which is never scoriaceous or porous, and these beds rise into chains 

 of bell-shaped hills. 



The syenitic greenstone is limited to a comparatively small space 

 at the middle of the formation, where it forms the foundation of the 

 bell-shaped elevations, being penetrated by dykes like the accompany- 

 ing rock. This formation is only disclosed in Fuertaventura, where 

 it forms no more than about one-fifth of the entire superficies of the 

 island, occupying the middle portion ; and, stretching for about twelve 

 miles along the north-west coast, it reaches its highest point on the 

 Atalaya peak, 2450 feet. 



The elevations of this formation are distinguishable at the first 

 glance, by their rounded wave-shaped outlines, from those of the other 

 formations, the hills of which are marked by cones and steep ridges. 

 If surprise be expressed as to the conjunction of syenite and trap in 

 one formation, Mr. Hartung remarks, that both at Palma and Grand 

 Canary, there occurs along with volcanic rocks of the oldest visible 

 formation a crystalline schistose rock compared by Von Buch to 

 granite ; and this formation is quite as distinct as in Fuertaventura. 

 In Madeira likewise, syenite is seen along with beds of agglomerate 

 and basalt in positions which show that the whole must there also 

 be called the oldest formation of the island. 



2. The oldest Basalt-formation consists partly of compact rock, 

 partly of scoriaceous deposits ; the whole arranged in such a way that 

 we must attribute their origin to the effect of a succession of eruptions 

 from cones. The lowest portion is not marked by layers ; appearing 

 to consist almost entirely of scoriaceous materials, and reaching the 

 surface only at the middle of the formation. Above is compact basalt 

 in beds, the thickest of which are columnar, the others more or less 

 vesicular, and alternating with scorise and layers of yellow or burnt-red 

 tufa. This system is inclined from the middle, where the subjacent 

 rocks are disclosed towards every point of the compass ; and amongst 

 it are found detached heaps of scoriaceous materials of limited extent. 

 The dykes strike for the most part in one direction, namely, in that of 

 the chain of hills. They are most numerous where the under scori- 

 aceous materials are most developed. Of the mountainous masses 

 once exhibited by this formation, only fragments remain ; a great por- 

 tion having been removed by the sea and by atmospherical influences. 

 Still, in many cases the original forms are still indicated, especially in 

 the peninsula of Handia, the highest point of which attains the eleva- 

 tion of 2770 feet. In the other part of the island of Fuertaventura, 

 the highest point of this formation reaches the altitude of 2240 ; 

 in the island of Lanzarote the height of 1860 feet. It would appear 

 that these oldest basalts formed a series of elevations and crests 

 through the longer axis of the island. On the side of the prevailing 

 winds the sea has in places removed large portions, and much has been 

 altered by weathering, assisted by rents caused by earthquakes or up- 



