Volcanoes, 4<31 



deposited by the fly above ground : it may be said, indeed, 

 that the ovipositor of the parent fly is constructed in a manner 

 capable of great elongation, so as to enable it to protrude its 

 eggs to a distance beneath the ground, and to fix them there 

 upon the outside, or even beneath the outer coats of the bulb. 



These, however, are all points of natural history, which a 

 minute series of observations can alone determine : and my 

 chief object, in thus noticing them, is in the hope that some 

 experienced agriculturist will take up the enquiry. It is this 

 class of the community who are the greatest sufferers from 

 this and similar evils ; it is not too much to hope that they 

 will consider the subject worthy of investigation.* 



The Grove, Hammersmith^ July 30. 1834, 



Art. VII. Volcanoes, By W. M. Higgins, Esq. F.G.S., Lecturer 

 on Natural Philosophy to Guy's Hospital. [Concluded from 

 Vol. VI. p. 350.] 



The Cause of Volcanic Activity, — There are few persons 

 who agree as to the cause of volcanic activity ; for there are 

 so many difficulties connected with the investigation of it, 

 that it may be fairly doubted whether we have sufficient 

 data to warrant the adoption of any one theory more than 



* [In the Gardener's Magazine, vii. 90 — 92., there are brief notices of 

 papers which had been reported on at the eighty-ninth meeting of the Royal 

 Prussian Horticultural Society. From one notice I quote that " Herr 

 Borggreve also confirms, by experience, the good effect of sprinkling pul- 

 verised charcoal over beds destined for onion seed. The mixing of char- 

 coal powder with the superficial mould, to protect bulbous roots against 

 the larvae of a fly (Anthomyia ceparum) has already been recommended 

 in the Transactions of the Society.*' These Transactions are entitled Ver- 

 handlungen des Vereins ziir Beforderung des Gartenhaues in den Koniglich 

 Preussischen Staaten, of which the first part of the tenth volume is pub- 

 lished : they are in quarto, and some insects troublesome in gardens are 

 figured and described in them. Growing plants of the cultivated onion 

 are occasionally eaten by other species of insects. In a bed of about 7 ft. 

 long, by 3i ft. broad, sown with onion seed in 1833, so late as early in May, 

 a plentiful crop of onion plants arose ; of the first number of which scarcely 

 more than half remained on September 12., the absent plants having been 

 successively eaten off at, and just below, the surface of the soil, by larvae 

 of some species of iVbctuadae, resident in the soil. Of these larvae, I found 

 within the area of the bed, on September 14., on digging up then the 

 remainder of the onions, forty-seven, most of them full grown, but some 

 of them not so. The late sowing of the seeds from which the onion plants 

 had arisen had occasioned them to be in a growing or vernal state, that is, 

 without bulbs and maturity, through much of the period named, and thus 

 had, perhaps, preserved them as eligible food for the iVbctuadae, when 

 earlier-sown, forwarder, and riper onions would not have been. The 

 bulbs of those left till the last were very small.] 



