SOME EXAMPLES OF ROCK VARIATION 383 



stituents : pyroxene (monoclinic and orthorhombic) , olivine, 

 hornblende, and biotite. Associated with them are feldspar, 

 apatite, green and brown spinel, and iron oxide. The relative 

 proportions of these minerals differs much, and yield different 

 well-known rocks. The types are not sharply separated, but 

 are found, both in the field and under the microscope, to grade 

 into one another. The purest form of peridotite is the wehrlite, 

 composed essentially of olivine and augite. Where, besides these 

 minerals, hornblende is present in large quantities, the rocks 

 belong to the amphibole peridotite type and approach Williams' 

 cortlandtite. In some specimens the biotite is almost in suffi- 

 cient abundance to warrant the naming of the rock biotite-peri- 

 dotite. Again, feldspar is present in comparative abundance 

 and the rock is a feldspathic wehrlite, and approaches an olivine- 

 gabbro or an olivine-hornblende-gabbro. 



A number of exposures of the peridotites have been exam- 

 ined, but only one will be described here, and that has already 

 been referred to under the gabbros. This peridotite cuts the 

 gabbros. Sections made from different specimens taken from 

 the exposure would be named, if considered separately, amphi- 

 bole-peridotite, wehrlite, or even olivine-gabbro. This is the 

 same exposure from which was taken a specimen described by 

 Patton ' as hornblende-picrite. In this rock there occur the 

 following essential mineral constituents, given in order of crys- 

 tallization : augite and olivine, apparently contemporaneous, 

 orthorhombic pyroxene, hornblende, biotite and feldspar. 



Of the mineral constituents forming the rock, augite is the 

 only one which is automorphic, and then only when it is par- 

 tially or wholly surrounded by feldspar. The olivine is in 

 rounded individuals which are never associated with the augite 

 in such a way as to enable their relative periods of crystalliza- 

 tion to be determined. Orthorhombic pyroxene, apparently 

 bronzite, occurs in grains inclosed in hornblende, and also forms 



" x Microscopical study of some Michigan rocks, by H.B. Patton, in Sketch of the 

 geology of the iron, gold, and copper districts of Michigan, by M. E. Wadswoeth, 

 Rept. State Board Geol. Surv. for 1891, 1892, 1893, p. 186. 



