392 J. CLIFTON WARD ON THE MICROSCOPIC STRUCTURE OF 



larity within the range of such variations can give no support to the 

 opinion that there is an essential difference between rocks of differ 

 rent ages." 



On June 24th of this year (1874) an important memoir was brought 

 before the Geological Society by Mr. Allport " On the Microscopic 

 Structure and Composition of British Carboniferous Dolerites." In 

 the October number of the Geological Magazine is an article by Prof. 

 Hull on the microscopic structure of the intrusive " porphyrite " 

 (quartzless porphyritic felstone) of the Island of Lambay, and in 

 the same magazine a supplementary note by Mr. Allport on the 

 Phonolite from the " Wolf Rock." 



Mr. Rutley, of the Geological Survey, has contributed notes upon 

 the microscopic structure of certain trap-rocks in Somerset to the 

 official memoir on the East-Somerset and Eristol Coal-fields, now in 

 the press. Also in the Survey Memoir on 101 S.E. (Keswick), in 

 the press, the author of these pages has given a detailed account, 

 with figures, of some of the volcanic and other rocks around Keswick. 



The preceding is, I believe, a fair notice of all that has been at 

 present done in our country towards the microscopic investigation of 

 volcanic rock-structure. Dr. Macalister, in his Presidential Address 

 to the Royal Geological Society of Ireland, has given a summary of 

 of work done by German penologists in this and other branches of 

 rock-microscopy. 



The following works and papers bearing on the microscopic struc- 

 ture of Yesuvian lavas, have been published on the Continent. 



In Zirkel's ' Mikroskopische Beschaffenheit der Mineralien unci 

 Gesteine,' 1873, and in his * Untersuchungen liber die mikrosko- 

 pische Zusammensetzung der Basaltgesteine,' 1870, will be found 

 notices of the microscopic structure of the lava-forming minerals ; 

 also in ' Mikroskopische Physiographie,' 1873, by Rosenbusch, 

 where a list is given of the literature of rock-microscopy. Kreutz, P., 

 " Mikrosk.Untersuchung der Yesuv-Laven vom Jahrel868," Sitzungs- 

 berichte der k.-k. Akademie der Wissenschaftenzu Wien, lix. 1869, 

 Eebruar. Inostranzeff, A. von, " Ueber die Mikrostructur der Yesuv- 

 laven vom September 1871, Marz und April, 1872," in Tschermak's 

 ' Mineralogische Mittheilungen,' "Wien, 1872, ii. ; Neues Jahrb. 1872, 

 883. Euchs, C. W. C, " Die Laven des Yesuv," Neues Jahrb. I860, 

 667; 1869,42,169. 



II. Examples op Microscopic Rock-structure AMona modern 



Lavas. 



1. Trachyte, Solfatara, near Naples. 



The rock first to be examined belongs to the bulky trachytic lava 

 erupted in 1198. Scrope describes it thus * :— " A highly crystalline 

 or granitoidal ash-grey trachyte, or rather greystone, made up of 

 large crystals of glassy felspar imbedded in a felspathic and granular 

 base, with a small quantity of augite." 



* Volcanos, 2nd edit. p. 321. 



