590 /. E. SPURR 



the extinction angles of andesine. In this section the pheno- 

 crysts are less in amount than the groundmass. 



Hornble?ide-andesite (ii8 N.).^ — -In the hand specimen this has 

 the appearance of a diorite closely verging upon diorite por- 

 phyry. Under the microscope, however, there is found to be, 

 between the closely packed phenocrysts, a micro-granular 

 groundmass, slightly finer grained than that of ii6 N. It con- 

 sists chiefly of feldspar with some hornblende. The phenocrysts 

 also are chiefly feldspar, with subordinate hornblende, which is 

 largely altered to epidote. Determinations of the feldspar show 

 that it is oligoclase. The larger phenocrysts have an average 

 diameter of about 2°"". 



Hornblende- quartz -ande site (i2i N.). — This rock is similar in 

 appearance to Ii8 N. It has also a fine holocrystalline granular 

 groundmass, showing no fluxional arrangement and composed of 

 feldspar and quartz. The phenocrysts have a maximum diameter 

 of about 3™"^, and consist of feldspar, green hornblende, and 

 subordinate quartz, the last named in large rounded or corroded 

 crystals. The hornblende is largely altered to epidote. The 

 feldspar, tested twice by the Fouque method, shows the extinc- 

 tion angles of andesine and andesine-oligoclase. 



Honiblende-quartz-andesite (i20 N.). — This rock in the hand 

 specimen is dense, greenish and fine grained, having a trap-like 

 appearance, with small phenocrysts in an aphanitic groundmass. 

 Under the microscope the rock is seen to be much like those 

 just described. The groundmass is fine holocrystalline, consist- 

 ing of lath-shaped feldspars and hornblende, with subordinate 

 quartz and some pyrite and siderite. There is a slight fluxional 

 arrangement. The phenocrysts belong to a distinct generation 

 from the groundmass {i. e., there are no transitions) and consist 

 of feldspar, pale green hornblende, and a single large grain of 

 corroded quartz. The phenocrysts reach a diameter of 2™°^. 

 The hornblende is largely altered to calcite, chlorite, and 

 epidote. 



Hornblejtde-andesite (iig N,). — This is from the same bed as 

 120 N., and is simply a variation from it. In appearance it is 



