292 Scientific Proceedings, Royal Dublin Society. 



3 mms. long were found protruding from the abraided surface of 

 the block pumice. In the dust they rarely exceed 1 mm. in 

 length. The formation of the dust by mere pulverization of the 

 pumice seems a most likely supposition. 



With regard to the modus operandi of preparing this dust for 

 the microscope, it may be useful to note that while it was found 

 that mere shaking up with water, and pouring off before complete 

 settlement, served to remove the lighter fragments of pumice, 

 complete separation was only readily effected by the following 

 method : — 



Into a glass tube 1 m. long and about 4 cms. in diameter, 

 closed at one end and filled with water to the brim, the partially 

 cleansed dust is introduced and allowed to settle. A slip of glass is 

 now jDressed on the open end, and the whole rapidly inverted into 

 a shallow dish containing water. The denser particles descending 

 most rapidly through the column of water in the tube reach the 

 dish first. When the more slowly moving particles are observed 

 to have nearly attained the dish, a movement of the tube to one 

 side effects the desired separation. 



One and all the constituents of this ash present under the 

 microscope a spectacle of the most extreme interest and beauty. 

 The almost unlimited minuteness of the vesicular structure of the 

 pumice, the endless variety in form and colour of the amorphous 

 fragments, the exquisite perfection of form exhibited by the minute 

 crystals of magnetite, of pyrites, of pyroxene and of feldspar ; the 

 nebulae of enclosures in these last, and their delicate overspreading 

 lace-work of still adhering vesicular glass. Here are seen small 

 crystals enclosing many others, and these again, still others ; while 

 often with the symmetry of the crystal the vitreous inclusions, 

 amber-yellow, range in concentric zones in the translucent depths. 



But if this be a wonderful spectacle viewed by ordinary trans- 

 mitted light, the beauty and wonder of it are a thousandfold 

 increased when, rotated in polarized light, each feldspar crystal 

 reveals its molecular symmetry in flashes of changing colour beside 

 vhich the richness of the emerald, ruby, or saphire, would pale, 

 x'he abundance of twinned crystals add much to the effect : and 

 now, also, many fragments passed over before as less interesting 

 put in their claim to symmetry, and shine not less brightly than 

 those diiiplayiiig facet and angle. Many hundreds of square miles 



