748 e. mallet ox the p1ling-tjp of volcanic cones. 



Discussion. 



Prof. Judd said that the subject treated of by Mr. Mallet was one 

 of great importance in considering the phenomena of the formation 

 of volcanic cones. At the same time he remarked that the subject 

 had by no means been altogether overlooked by previous "writers. 

 The inward dip of some beds had been noticed in Icelandic volcanos, 

 and also by Mr. Darwin in some of those observed during the voyage 

 of the ' Beagle/ and described in his work. The same phenomenon 

 also occurred in New Zealand. Prof. Judd said that he had himself 

 shown in the case of the great volcano of the island of Mull that 

 the admission of this dip towards the centre is essential to the com- 

 prehension of the structure of that volcano, and calculated the amount 

 of change that has taken place in the inclination of the materials 

 of which it is composed. He also stated that he could find no 

 difference in the mode of volcanic action in older and more recent 

 times. 



The Author, in reply, said he perceived he had laid himself open 

 to the remarks which had been just made by not having sufficiently 

 distinguished in his paper between the effects due to evisceration 

 and those which he attributed to subsidence by the continually in- 

 creasing weight of material evolved from the base of a volcanic cone. 

 There was nothing new in the fact that evisceration must produce sub- 

 sidence; that had long since been noticed by writers on volcanos ; 

 and the depression thus produced may affect the form of the base of 

 the cone in any way. The point to which the author intended 

 specially to draw attention was, that the heaping-on of matter in 

 the production of a volcanic cone must alter the base in a definite 

 form, so that, assuming the cone to be deposited upon an originally 

 level plain, the effect of the imposition of the cone would be to de- 

 press the base into the form of a saucer or bowl. 



base of architectural structures, let us assume a round brick chimney stalk (such 

 as examples existing in the neighbourhood of Glasgow and elsewhere), the dia- 

 meter at the base being 40 feet, the thickness there 5 feet, and the height 

 4C0 feet, the batter and reduction in thickness with height being as usual in 

 Buch cases. Assuming the brickwork to weigh 112 lb. per cubic toot, the pres- 

 sure per square foot of base would be only about (> tons. But. as the designers 

 of such structures judiciously enlarge the area of base by external offsets and 

 other means until the area is doubled or more, so the actual pressure would in 

 this instance be only from 2 to 3 tons per square foot. Yet such lofty and 

 narrow structures as that here adduced visit a more severe stress upon their 

 base and foundations than probably any others within the whole range of 

 architecture and engineering. All reference is of course excluded here from 

 piled or other purely artificial foundations, such as are employed beneath the 

 piers of large bridges, &c. 



