348 Dr James Macaulay on the Physical Geography^ 



the iron contained in the mineral also becomes peroxidated, so 

 that the dark colour is changed to various hues of brown and 

 red. In regard to the position of the basalt, the greater part 

 exists in layers, or stratiform bed^, of different degrees of 

 thickness. These beds do not seem to follow any one general 

 direction, but lie in an irregular manner throughout the island. 

 They are also in various position with relation to the other 

 formations, being interstratified with some, overcapping others, 

 and appearing amongst others in the form of veins and dykes 

 and amorphous masses. In some places the columnar form is 

 assumed in great perfection. Of the basaltic dykes, some 

 magnificent specimens may be observed on the road to St Vin- 

 cente after passing the ravine of the Serra d'Agoa, where they 

 are seen rising through the whole of the formations to the 

 very summits of the mountains. 



Basaltic Conglomerates. — Basaltic conglomerates of various 

 kinds form the great mass of the mountains, and whole surface 

 of the island. These conglomerates resemble each other in 

 containing fragments of the same pyrogenous rocks, but pre- 

 sent an immense variety, according to the nature, size, and 

 quantity of the basaltic fragments which they contain, and the 

 proportions of the volcanic mud and other products of igneous 

 eruptions which enter into their formation. In some cases the 

 compact dark basalt predominates, existing in globular masses, 

 or in irregular fragments. In others the scoriaceous and cel- 

 lular structure is more prevalent. Sometimes the fragments 

 contained are so minute, that the conglomerate has a compact 

 and homogeneous texture. With the various conglomerates 

 the compact basalt everywhere alternates, and traverses them 

 by veins and dykes. The position of these formations may be 

 seen in the sections which are presented everywhere, both in 

 the interior and along the coast. In the magnificent sea-cliff 

 of Cape Giram, formerly referred to, various species of conglo- 

 merate, with basaltic dykes penetrating ^them, are finely dis- 

 played. The great mass of the high land of Madeira is of a 

 structure analogous to what is there seen. 



Along the sea-cliffs, towards Cape Grajam, various beds of 

 rocks are seen, which, from the manner of their stratification, 

 have evidently been deposited from aqueous suspension. They 



