276 Notice of Active and Extinct Volcanos. 



" It appears from this table that the nearest coincidence be- 

 tween the eruption of the two volcanos was in 1694 and in 181 1, 

 when they occurred within a month of each other ; and that on 

 eight several occasions an interval of less than half a year elap- 

 sed between them, viz. that of Vesuvius December 2, 1754, 

 was followed by one of Etna on March 2, 1755 ; Vesuvius Au- 

 gust 3, 1779, by Etna May 18, 1780; Vesuvius October 31, by 

 Etna July 28, 1787 ; Etna June, 1788, by Vesuvius February, 

 1799; again followed by one of Etna in June, same year; Etna 

 March 27, 1809, by Vesuvius December 10, 1809; Vesuvius 

 October 12, 1811, by Etna October 25, 1811 ; again followed by 

 Vesuvius December 31, same year; Vesuvius May 27, 1819, by 

 Etna, November 25, same year." 



The analysis already given, embraces all the countries in 

 which Professor Daubeny made personal observations and in- 

 cludes about half his volume. In the remainder we have a 

 notice of volcanos existing in countries not visited by the au- 

 thor ; and general remarks on volcanic phenomena. 



As it is our object not merely to display the uncommon merit 

 of Dr. Daubeny's performance, but by its aid to bring under 

 the eyes of our readers, a succinct and yet comprehensive ac- 

 count of volcanic formations both ancient and modern ; and 

 of the most important phenomena connected with them, we 

 will proceed upon the plan already presented. 



Iceland. 



Sir G. Makenzie, in his work on Iceland " notices two varie- 

 ties of volcanic products in this island, one of which appeared 

 to him of submarine, the other of terrestrial origin. 



" Among the rocks referred to the former period, the prevail- 

 ing substance was a tuff containing fragments of cellular lava, of 

 pearlstone, and of amygdaloid, the cavities of which were filled 

 with calcareous spar. With this tuff" alternate beds of scoriform 

 lava, and both are traversed by dykes of greenstone, perfectly 

 compact, and without any vitreous aspect, thus serving to shew 

 the manner in which the characters of a rock depend upon the 

 degree of pressure exerted during its formation. In the case of 

 the bed the structure is cellular, because it probably flowed free- 

 ly over the surface, without being subjected to any pressure con- 

 siderable enough to counterpoise the expansive force of the 

 elastic vapours disengaged ; in the case of the greenstone dykes, 

 the rock itself, through which they forced their way, may have 

 opposed a resistance sufficiently considerable to have prevented 

 the formation of cells." 



