chap. vi. Volcanic Islands, 143 



group, which, appears to be the least symmetrical of 

 any oceanic volcanic archipelago, a NW. and SE. 

 line formed by several islands, if prolonged, would 

 intersect at right angles a curved line, on which tlfe 

 remaining islands are placed. 



Von Bach l has classed all volcanos under two heads, 

 namely, central volcanos, round which numerous erup- 

 tions have taken place on all sides, in a manner almost 

 regular, and volcanic chains. In the examples given 

 of the first class, as far as position is concerned, I can 

 see no grounds for their being called ' central ; ' and 

 the evidence of any difference in mineralogical nature 

 between central volcanos and volcanic chains appears 

 slight. No doubt some one island in most small 

 volcanic archipelagos is apt to be considerably higher 

 than the others ; in a similar manner, whatever the 

 cause may be, that on the same island one vent is 

 generally higher than all the others. Von Buch does 

 not include in his class of volcanic chains small archi- 

 pelagos, in which the islands are admitted by him, as 

 at the Azores, to be arranged in lines ; but when 

 viewing on a map of the world how perfect a series 

 exists from a few volcanic islands placed in a row to 

 a train of linear archipelagos following each other in 

 a straight line, and so on to a great wall like the 

 Cordillera of America, it is difficult to believe that 

 there exists any essential difference between short and 

 long volcanic chains. Von Buch 2 states that his 

 volcanic chains surmount, or are closely connected 

 with, mountain-ranges of primary formation : but i£ 

 trains of linear archipelagos are, in the course of time, 

 by the long-continued action of the elevatory and 

 volcanic forces, converted into mountain-ranges, it 



1 ' Description ci.es Isles Canaries,' p. 324. 

 8 Idem, p. 393. 



