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University of California Publications in Geology [Vol. 13 



partially altered to green hornblende the whole area was considered 

 as one grain. Where isolated areas of ferromagnesian minerals 

 appeared to be parts of a poikilitic individual the diameter was taken 

 to be that of the entire area occupied by those grains with parallel 

 optical orientation. 



The average size of grains of the basic rocks, excepting the dike 

 rocks, was found to vary from 0.3 mm. to 1.2 mm. The average of 

 all cases is 0.57 mm. The maximum size of grain, as observed in thin 

 section, varies from 0.8 mm. to 13.0 mm., the average maximum of all 

 cases being 2.6 mm. 



Viewed in the hand specimen the rocks would be termed medium 

 to fine-grained. Even in those of finest grain, however, many of the 

 constituent minerals can be distinguished with the hand lens, and often 

 there are large luster mottled pyroxenes, in general less than 1 cm. 

 in size, but sometimes reaching 5 or 10 cm. 



The results of grain measurements under the microscope were sur- 

 prising, as the impression was gained from examination of the hand 

 specimens that the rocks were in general of medium grain, and that 

 some were even moderately coarse. The reason for this is to be found 

 in the mode of occurrence of the feldspar. In most of the rocks the 

 plagioclase shows little tendency to tabular or prismatic form, but 

 occurs in aggregates of small anhedral grains, set together with 

 smoothly curving boundaries. 



Iddings gives the following classification of the grain of rocks : 

 "Coarse-grained, average crystals greater than 5 mm. in diameter; 

 medium-grained, crystals between 1 and 5 mm. ; fine-grained, crystals 



is ladings, J. P., Igneous rocks, vol. 1 (1909), p. 192. 

 less than 1 mm." 15 According to this scheme, the rocks of the Cuya- 



maea basic intrusion are for the most part fine-grained. 



For comparison a few gabbros and norites from various European 



localities were studied, with the following results: 



Number of specimen 74 77 78 79 81 



Krantz-Rosenbusch Coll. 



Average grain , 4.3 mm. 1.3 1.5 1.5 1.2 



Maximum grain 10.0 mm. 3.2 4.0 4.0 6.0 



No. 74, Norite, Hitteroe, Norway. In the hand specimen appears medium, 

 verging on coarse-grained. 



No. 77, Olivine gabbro, Oberkainsbach. Medium-grained in hand specimen. 



No. 78, Olivine gabbro (Hyperite), Risor, Norway, appears medium-grained, 

 verging on coarse. 



No. 79, Olivine norite, Radantal, Harz Mountains, appears medium-grained. 

 No. 81, Troctolite, Radantal, Harz Mountains, appears medium-grained in 

 hand specimen. 



