ANNIVERSARY ADDRESS OF THE PRESIDENT. H 



of the mountain, which seems to indicate a falhng in of a portion of 

 the summit. The mountain, he says, is a very regular cone, with a 

 slope of from 25 to 30 degrees ; the height of the loftiest part they 

 estimated by barometric measurement at 1400 metres (4593 English 

 feet), but this he says is 157 metres (515 feet) less than former tri- 

 gonometrical measurements ; and although, from some defects in their 

 means of observation, he considers 1400 metres only as an approxi- 

 mate height, he does not think that the error, if there is one, can have 

 amounted to so much as 1 57, and that therefore there has been a consi- 

 derable breaking down of the sides of the crater. Sir George Mac- 

 kenzie describes '^ the middle peak " of Hecla as forming one side of 

 a hollow, evidently a crater ; adding that the whole summit is a 

 ridge of slags, and that the hollows on each side appear to have been 

 so many different vents from which the eruptions have from time to 

 time issued, but that they saw no indications that lava had flowed 

 from the upper part of the mountain * . According to M. Descloizeaux, 

 the crater at the summit is almost circular, with an external talus of 

 scoriae, having an inclination of from 33 to 35 degrees ; thus it is 

 evident the form has changed since the visit of Sir George Mackenzie 

 and his companions. Dr. Holland and Dr. Bright, and it is probable 

 that the higher sides have fallen in, thus accounting for the diminution 

 of height. The exterior part of the cone M. Descloizeaux describes as 

 traversed by fissures containing fumeroles which deposit sulphur ; and 

 as the bottom of the crater was covered with old snow, it was clear 

 that the eruption of 1845 was not from the main crater, but, like the 

 more recent ones of Etna, from the side of the mountain. On one 

 side M. Descloizeaux observed two craters connected by a very narrow 

 ledge, one of them 600 feet in diameter, and the other half that di- 

 mension. * He does not give the height from which the eruption took 

 place, but describes the stream of lava that was poured forth to have 

 been directed W.S.W. From the place where it burst forth, to its 

 termination in the plain below, he estimates its length to be 16 kilo- 

 metres, which is nearly equal to 1 English miles ; its greatest breadth 

 at 2 kilometres (about \\ mile) ; and its thickness ranges, he says, 

 from 15 to 25 metres ; that is, from 49 to 82 feet. This is an 

 enormous mass, but it is insignificant in comparison with that which 

 flowed from the neighbouring mountain of Skaptar Jokul in 1783; 

 there were then two streams, one 50 miles in length, with a breadth 

 of from 12 to 15 miles, the other 40 miles long and 7 miles broad, 

 both 100 feet in thicknessf . But in regard to this stream of 1845, 

 there is an important fact communicated by M. Descloizeaux, viz. 

 that they found the inclination of the stream very variable through- 

 out, " from degree to 25° " — an observation of great interest as 

 regards the theory of the formation of volcanic mountains. He further 

 describes its structure as follows : — " The stream is in no part homo- 

 geneous ; it consists throughout of isolated blocks, often of very con- 

 siderable volume, accumulated mth a certain degree of symmetry, the 

 congeries resembling an immense ribbon, at the edges of which is a 

 talus, with an inclination between 35 and 40 degrees ; and the interior 



* Id. page 248. f Lyell's Principles of Geology, 7th Ed. p. 408, 



VOL. III. e 



