54 Scientific Proceedings, Royal Dublin Society. 



implanted on quartz only, as is common with normal beryls from 

 Grlencullen. 



I had a section from one of these crystals prepared for the 

 microscope by Mr. Gregory, of London ; it was, by my directions, 

 taken parallel to one of the prismatic faces. The specimen was in 

 colour mottled green and white, with some rusty marks. 



On examination in the polarizing microscope it appeared, in 

 the first place, that more than one mineral entered into the com- 

 position of these crystals. The fundamental constituents were 

 evidently two in number. There was. a constituent presenting the 

 appearance of a felspar, and there was a more homogeneous con- 

 stituent, which I suspected to be beryl. These were mixed, archi- 

 pelago-like, in wild confusion, but always quite distinct. The 

 felspathic part extinguished locally or in plumed shadows, which 

 crept over the field as the stage was rotated. Faint cross-hatching, 

 checquered or wavy marks, recalled microcline : these marks appear 

 in fig. 3, pi. in. Such are, however, common in well-authenti- 

 cated orthoclase. It showed, too, the habitually quiet colours of 

 that felspar, slate-grey in this case ; and in fact I had little doubt 

 it was orthoclase. Lately, however, examination of the cavities 

 eaten in these crystals by decomposition has set the question at 

 rest. In these, bunches of small laminate crystals, resembling 

 white orthoclase in appearance, branch from the walls in tufts and 

 plumes; their grouping suggestively recalling the plume-like 

 extinctions obtained on the sections. Fragments of these tufted 

 crystals, removed and placed in a diffusion zone above Thoulet's 

 solution, according to the simple and accurate method devised by 

 Professor Sollas, float side by side with the Grlencullen orthoclase. 

 Their specific gravities are, therefore, identical. Again, when 

 compared with Grlencullen orthoclase on the Meldometer their 

 melting points are found to be identical. There is little doubt, 

 then, that this constituent is orthoclase. 



It is seen at once on the section that this orthoclase includes a 

 great many sharply-defined, brilliantly polarizing crystals, present- 

 ing a very beautiful appearance. They are very small, and, with 

 great probability, are iolite. 



In the second principal constituent extinction is not local, but 

 takes place simultaneously all over the field, leaving the felspar 

 standing out in luminous veins and patches. — Fig. 1, pi. n. (x 18 



