AND PORTO SANTO. 25 



probably, not formed at a very considerable depth beneath the 

 surface of the globe), and it was always supernatant'. If I am not 

 mistaken, pumice has not hitherto been found with basalt ; when 

 I picked up a detached morsel on my landing, it led me to expect 

 a trachytic formation : I do not think there is a trace of obsidian 

 in the island. The scorias, especially in the inland sections, are 

 frequently coated with a shining matter, generally pale brown, but 

 sometimes black, and of a bituminous appearance ; it did not 

 detonate however with nitre, nor did it lose its colour or lustre at 

 a red heat. A grey crustaceous hchen (idiothalames Ach.) covers 

 the porous and compact basalt, (in patches, ring within ring) and 

 is generally accompanied by another, equally crustaceous, but more 

 delicate in form, and of a deep orange. A third forms large light 

 coloured patches on the inland basaltic rocks, and is so abundant, 

 that in several instances it gives a different hue to that part of the 

 landscape". All the lichens of Madeira are extremely interesting, 

 from their abundance or beauty; but, for the before-mentioned 

 reason, I have only been able to refer some of them to the great 

 divisions of Acharius. The anethum starts out from the rocks in 

 the same way as at Lisbon, and is found in great quantities on the 

 sea-shore; the ferula glauca is abundant. The only species of 

 cactus which can decidedly be pronounced indigenous, is the 

 c. opuntia, which only grows on the rocks nearest the sea™. 



' M. Guillin, after a mere glance at Funchal and its bay, has not hesitated to assert, 

 " que la lave qu'on trouve a Madere n'a aucune partie vitrifiee, ni aucune veritable 

 pierre ponce." See the Appendix of the Voyage de Bory St. Vincent. 



" Genus, thallus crustaceus, pallidus. Scutella albce, in thallo centralis. 



" The recent shells which I found scattered over the black ferruginous sand, and 

 amongst the basaltic pebbles of the beach, were a inurex, a tritonia ? (brown, with 

 darker stripes, and yellow lips) ; murex, (white) ; a purpura, (dusky brown) ; a colom- 

 bella, (white) ; a broken specimen of the argronauta tuberculosa, and an avicula, hght 

 brown, mottled with black towards the beaks. The patella plicata abounds on the 



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