42 NEW YORK STATE MUSEUM 



In connection with this work a number of thin sections of other 

 garnet occurrences in the Adirondacks were studied and, in part, 

 photographed. These sections have been supphed by Professor 

 Kemp and have been in part previously described by him. 

 Plate i8B illustrates a reaction rim in the basic gabbros. It intror 

 duces biotite. Plate 19 contains two cases of garnet growths from 

 the contact zones produced from limestones by the intrusives. The 

 upper illustration (A) is taken from a slide from the same exposure 

 as is illustrated in plate 8. Narrow zones of garnet have developed 

 between the pyroxene (hedenbergite) and scapolite. The lower 

 illustration (B) brings out a very peculiar interfingering or parallel 

 growth of pyroxene and garnet, from the Weston mine. Nothing 

 in the nature of a rim is presented by the slide, but rather the intimate 

 relations which the production of pyroxene in a limestone contact 

 bears to that of garnet. Similar intergrowths of garnet in the multi- 

 ple-twinned plagioclase, the garnet taking the place of alternate 

 lamellae have been described by Professor Kemp.^ Plate loB illus- 

 trates the formation and relations of garnet in a gneiss of the gen- 

 eral composition of the syenites and believed to belong to them. 

 While the feldspar is predominantly orthoclase, acidic plagioclase 

 also enters, and probably contributed to the garnet. 



Discussion. Upon the basis of the evidence presented and upon 

 the observations of others a satisfactory explanation of the reaction 

 rims must be based. Search through the literature revealed the fol- 

 lowing. The earliest reference to a case at all similar is given by 

 A. Lacroix in a paper on " Gneiss a Pyroxene et a wernerite de 

 Bretagne," printed in the Bulletin de la Societe Mineralogique de 

 France, v. 12, 1889, p. 85-365. In giving a description of some 

 rims around olivine he refers to an article by Tornebohm on 

 some rims about olivine in the gabbros of Wermland.^ 



The original paper of Tornebohm's was not available but, accord- 

 ing to the citation in Lacroix, it dealt with rims of other minerals 

 than garnet. Lacroix himself describes, among others, some rims of 

 garnet, between labradorite and amphibole which surround pyroxene 

 arormd biotite around a core of magnetite. These rims occur in a 

 gabbro at Odegarden, which is in contact with an amphibole gneiss. 

 The only comment made in regard to origin of the rims is " Dans le 

 roches que nous etudions, il semble difficile d'admettre que le grenat 

 soit exclusivement forme au contact du gneiss amphibolique." 



1 Kemp, J. F., " Gabbros on the Western Shore of Lake Champlain." Bui. 

 Geo!. Soc. Am., 1895, 5 '-219-20. 



^ Kongl. Svenska Vetensk. Akad. Fordhandl, i. Stockholm, 1877. 



