70 H. F. Reid — Studies of Midr Glacier. 



and spheres, which grow out from the angles of the porphvritic 

 cr3^stals. 



Quartz-pwphyry. — Tlie one specimen of tlie collection which 

 may be classified without doubt as a quartz-por2:)hyry or rhyo- 

 lite is number 12 of Professor Reid's collection. This consists of 

 a 3^ellowish-white lithoidal grounclmass, enclosing sharj) crystals 

 oi bipyramidal quartz and orthoclase. Under the microscope 

 the groundmass appears to be holocrystalline and obscurely 

 granular. It is full of minute kaolin flakes, due to incipient 

 alteration. The quartz crystals have their forms corroded with 

 characteristic embayments of the groundmass. They are sur- 

 rounded by a 3^ellow stain. This rock is without doubt acid 

 enough to belong to the rhyolite series, and it has a strong macro- 

 scopic resemblance to certain rhyolites; still, in accordance with 

 the principles set forth in the foot-note on page 65, we may more 

 consistently call it a quartz-porphyry on account of the holo- 

 crystalline character of its groundmass. 



Porphyrite. 



Hornblende-porphyrite. — By iar the greater proportion of Pro- 

 fessor Reid's tine-grained rocks contain amphibole as their only 

 original ferro-magnesian constituent. The amount of this min- 

 eral present is usually very scanty', while l)oth it arid the feld- 

 spars are so altered and decomposed as to be hardl}^ recognizable. 

 Some dozen or more of these specimens, of which thin sections 

 were made for study, are so uniform in structure and composition 

 that they might readily have been derived from a single geologic 

 soilrce, while their weathered condition deprives them of any 

 special interest or individuality. 



One of these sijeciniens, however, number 32, is in this respect 

 an exception. It is a brownish-gray rock of typical andesitic 

 habit, which is thickly studded Avith small white rectangular 

 feldspars. Under the microscope these plagioclase phenocrysts 

 are found to l)e peripherally and sometimes entirely altered to 

 calcite. The groundmass of this rock is its most interesting 

 feature. This consists of a rather coarse aggregate of idiomorphic 

 or hypidiomorphic feldspar laths and crystalloids of brown horn- 

 blende. These are Avell developed in their prisna-zone but are 

 Avithout terminations. They are someAvhat altered to chlorite, 

 but on tlie Avhole are remarkably Avell jn-eserved. Magnetite is 



