GEOLOGICAL EBOBLEM. 
103 
Terse to the rapidity desirable in an itinerary ; but it ap- 
pears to mo tliat, in a narrative, the principal end of which is 
the progress of physical knowledge, every oilier consideration 
ought to be subservient to those of instruction and utility, 
lly isolating facts, travellers, whose labours are in every other 
respect valuable, have given currency to many false ideas of 
the pretended contrasts which Nature oilers in Africa, in 
New Holland, and on the ridge of the Cordilleras. The great 
geological phenomena arc subject to regular laws, as well 
as the forms of plants and animals. The ties which unite 
these phenomena, the relations which exist between the 
varied forms of organized beings, are discovered only when 
we have acquired the habit of viewing the globe as a great 
" hole ; and when we consider in the same point of view the 
composition of rocks, the causes which alter them, and the 
productions of the soil, in the most distant regions. 
^ Having treated of the volcanic substances of the isle of 
1 enerifle, there now remains to he solved a question inti- 
mately connected with the preceding investigation. Does 
tlie archipelago of the Canary Islands contain any rocks of 
primitive or secondary formation ; or is there any production 
observed, that Las not been modified by lire ? This interest- 
ing problem has been considered by the naturalists of Lord 
Macartney’s expedition, and by those who accompanied cap- 
uun Baudin in his voyage to the Austral regions. Their 
'-'pinions are in direct opposition to each other; and the 
contradiction is the more striking, as the question does not 
refer to one of those geological reveries which we are ac- 
customed to call systems, but to a positive fact. 
Doctor Gillan imagined that he observed, between Laguna 
snd the port of Orotava, in very deep ravines, beds of primi- 
tive rocks. This, however, is a mistake. What Dr. Gillan 
vails somewhat vaguely, mountains of hard ferruginous clay, 
are nothing but an alluvium which we find at the foot of 
every volcano. Strata of clay accompany basalts, as tufas 
accompany modern lavas. Neither M. Oordier nor myself 
observed in any part of Teneriffe a primitive rock, either 
p natural place, or thrown out by the mouth of the 
oak ; and the absence of these rocks characterizes almost 
every island of small extent that has an un extinguished 
v olcano. W e know nothing positive of the mountains of 
