272 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Ratio, Albite to 

 Microcline — 



1.30 

 Albite 56.3 

 Mic. 43.7 

 100.0 



The low magnesia indicates that tlie pyroxene is largely a Hme and 

 ferrous iron — rich member of that family (augite-hedenbergite). It is 

 interesting to note that the proportion of Or: Ab + An as deduced by 

 Vogt*^ for the cryptoperthites, (anorthoclase) of the rhombenporphy- 

 ries associated with the Laurvikite of South Norway is 42: 58, which 

 is very near that found here viz. 43.7:56.3. With the exception of 

 the silica percentage, high total alkalies, and the relative proportions 

 of the alkalies present in the feldspar, there appears to be little re- 

 semblance chemically between this rock and the rhombenporphyry 

 whose analj'sis appears above. Nevertheless the rock, texturally and 

 mineralogically, belongs to this type and its occiu'rence with the alkali- 

 granites of this area is interesting and significant. 



Cognate Xcnoliths. 



Patches differing in texture and usually of darker color, are 

 characteristic of the coarse-granite and its porphyritic phase and 

 of the porphyry along the deeper contacts. They are without excep- 

 tion derived from the magma by some process of differentiation and 

 are believed to represent for the most part, if not entirely, fragments, 

 perhaps somewhat modified, derived from marginal facies, immersed 

 and frozen in the consolidating magma. Following Harker *^ we shall 

 call these masses cognate xenoliths. As has been already noted, they 

 are especially abundant in those parts of the field where the contact 

 porphyry is thin, and where also the more basic, feldspar-porphyry is 

 developed — that is, in portions of the field which represent relatively 

 deeper parts of the contact zone, where the magma was less extensively 

 chilled, and where differentiation could take place. They are like- 

 wise strongly developed in the porphyritic phase of the granite imme- 

 diately underneath the heavy cover of granite porphyry (as on the 



41 T. H. L. Vogt, T. M. P. M., 24, No. 6, p. 524 (1906). 



42 National Hist, of Igneous Rocks. A. Harker, p. 347 (1904). 



