Minerals in Dacltr. 529 



representiitive .sainplf „r the iioruml dacite in that area. It is a 

 dark-grey, and very hard compact porphyritic rock. In the 

 hand specimen phenocrysts c.f a. dark-brown mica, biotite, in 

 well-formed hexagonal Hakes, can be seen, also phenocrysts of 

 felspar, but further than tliar little else can be observed mega- 

 scopically. Under the miscroseope in thin sections one can see 

 abundant phenocrysts of plagioclase, generally with regular out- 

 lines, and very often zoned, dark-brown biotite, pale-green 

 hypersthene, quartz very occasionally, and an opaque mineral 

 which may be either magnetite or ilmenite. all set in a fine- 

 grained crystalline groundmass of quartz, felspar and biotite. 



Of the minerals in the rock the plagioclase felspars can be 

 determined optically by the Michel-Lovy method, and so definite 

 compositions assigned to them ; but it is necessary to separate 

 and chemically analyse the minerals biotite, hypersthene, and 

 the opaque mineral in order to learn their compositions. ' 



The proportions of the phenocrysts and groundmass are 

 about equal, and as the average diameter of the former is only 

 .1 mm., it was seen that the separation of them in sufficient 

 quantity and in a pure enough state for a chemical analysis 

 would be a matter of some difficulty. 



Methods of Separation Adopted. 



The minerals to be separated are all iron-bearing, so that 

 the electro-magnet suggested itself as a means to the desired 

 end ; accordingly one as described by T. Crook,i of the Imperial 

 Institute. London, was used. 



By a combination of the electro-magnet with the use of heavy 

 li-iuids and various shaking devices, and finally picking with a 

 wet brush underneath the microscope, the niinerals were ob- 

 tained pure enough for chemical analysis. 



The electro-magnet consists of a cylindrical piece of soft iron, 

 1 inch in diameter, bent into U shape and having vertical limbs 

 about 6 in. long and 3 in. apart. Each limb i.s provided with 

 a Ijobbin on which is wound seven layers of insulated wire (16 

 gauge), each layer having about iO turns. Two pole pieces, 



1 "The Use of the Klect.ro-Ma-r.et in Petrography,- hr T. Crook, A.R.C Sc I F G S 

 Science Progress, No. 5, .lulv, 1907. ' • •• ■ 



10 



