12 RECORDS OF THE AlISTT!Af,lA\ MTSETM. 



that an increase in t lie amount of alkalies, especially caesium, contained 

 is accompanied by a rise in specific gravity, in refractive indices and 

 bii-efringence we should expect the Torrington beryl to contain a low 

 percentage of alkalies and to have a low specific gravity. As will be seen 

 presently this expectation is fulfilled. 



I 'o]iipositioii and Sjifcilic (Jnirlfii. — Two analyses of Torringtoii bei'vl, 

 probably from HefFei-iian's Mine, have been made by Mr. .1. (•. H. 

 Mingayel'' witli the following i-esults: — 



I. II. 



6(;-08 G-t-70 



19 86 ... 2-1-22 



12-5)6 ... 11-84 



OHO ... ... ()-37 



ir. ti-. 



abs. . abs. 



„ 004 



0-43 0-24 



abs. . 013 



pres. .. pres. 



0-58 ().()0 



abs. nl)s. 



100-21 100-14 



S. (J. ... 2-661 2-666 



Analyst's Note. — Caesium, if present, is in very small amount ; 

 sti'ong spectroscopic i^eaction for liMiiuni jind strontium. 



The specific gravity of the crystal from which the section was cut for 

 optical examination was determined; the fragment weighed 3167'.' gi-ains 

 and its specific gravity, found b}' the oi-dinary weighing method, is 2-664. 



]'t(r(((ieui'!iis. — From a stud}' of hand specimens and the observations 

 made at HeiTeinan's Mine, and the Torrington district generally, by 

 Carne, Andrews, and liogeniiedei-, it is clear that the beryl was formed 

 in a pegmatitic phase of a granitic ro(d<. Mr. Carne'" who has made a 

 caiet'iil study of the oie occuj-rences at. Torrington, is of opinion that the 

 oie rock, which contains wolfram, qnai'tz, felspa?-, mica, beryl, and toi)az, 

 is the latest [)hase of granitic intrusion into the sedimentaiy locks. It 

 i.s conceivable that thei'e were successive invasions by mineralising 

 vapours fi-om the cooling magma as this would perhaps account for the 

 marked zonal structure of the quartz and beryl crj'stals. Constructive 

 pneumatolysis was followed by ih'stnictlve pneumatolysis, the evidence for 

 which is seen in tlie etching of the quartz and beiyl and in tlie 

 accumulation of what Mi-. C^ariie calls decomposed inicax^eoiis i-ock and 



"' Min},'aye— Ann. Rwpt. Dept. Mines N.S.Wales, 1915 (191(51, p. 198. 

 '• Carue— Dcpt. Mines, N.S.Wales: Min. Kes., Nu. I."i. I'.Ml'. i.. l-O, 



