PERKNITE (LIME-MAGNESIA ROCKS)' 



There are sometimes associated with diorites, gabbros and 

 peridotites, dark rocks composed largely, or entirely, of mono- 

 clinic amphibole or pyroxene, or both. These rocks differ 

 mineralogically from diorites and gabbros, in containing little 

 or no feldspar, and from peridotites in containing rhombic 

 pyroxine or olivine in relatively small amount, if present at all. 

 Chemically these rocks contain less alumina than diorites and 

 gabbros, and less magnesia than peridotites. They are low in 

 alumina and in the alkalis, moderately rich in lime, magnesia, 

 and the iron oxides. 



The chief constituents of perknite are monoclinic amphibole 

 and monoclinic pyroxene ; the secondary constituents rhombic 

 pyroxene, olivine and feldspar; the accessories biotite, iron ore, 

 etc., but only one of the primary constituents may be present 

 with none of the secondary constituents or accessories. The 

 existence of this group of rocks has long been recognized, but 

 from their occurrence usually in small masses, and from the fact 

 that many of them are of simple composition so that the self- 

 explanatory names pyroxenite and amphibolite or hornblendite 

 have answered, they have never been grouped together under 

 one name. 



In the State of New York^ and in California^ there are rocks 

 containing both monoclinic pyroxene and amphibole as principal 

 constituents, and doubtless this is likewise the case in many 

 other parts of the world. Moreover, in California such rocks 

 form areas of geological importance. There is, therefore, some 

 reason in grouping all of these lime-magnesia rocks under a 

 common name. It is proposed to call the group perk?iite from 



' Published by permission of the Director of the U. S. Geological Survey. 

 » G. H. Williams : Am. Jour. Sci., Vol. XXXI, 1886, p. 40. 



3 Turner : Am. Jour. Sci., Vol. V, 1898, p. 423. Turner and Ransome. Sonora 

 folio. 



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