of the North of Ireland. 233 



APPENDIX. 



On the Dykes of Monte Sommay in Italy, 



Extracted from a Series of Letters addressed to the late 



LORD CHARLEMONT, P.R I.A. 



By the late Rev. GEORGE GRAYDON, Fellow of Triuity College Dublin. 



Dated Naples. 



" The appearance of the face of the cliff of Monte Somma in its 

 whole length perfectly coincides with the idea given by its semi- 

 circular shape, surrounding Vesuvius, as well as by its sloping back 

 as seen from Naples, and strongly confirms the opinion of its 

 having once formed a much higher conical hill." 



" The face of the cliff viewed on a horizontal line is by no means 

 smooth, but considerably indented in some places into semicircular 

 or more than semicircular hollows or recesses, with sides nearly 

 perpendicular ; in others into hollows with steep sloping sides 

 down which the sand and stones are continually sliding." 



" It is foimed of a great number of successive strata of lavas, in some 

 places perhaps upwards of thirty ; these strata in general are thin, 

 that is, not exceeding from three to six feet in depth of solid stone, 

 the intervals between them, which are generally much thicker than 

 the strata themselves, consist of porous red or calcined and usually 



* This gentleman presented a series of volcanic specimens, collected by himself on 

 Vesuvius, to the Royal Irish Academy; vide Kirwan's Mineralogy, ed. 1794, vol, 1. 

 preface, page xv, where he is much commended. 



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