PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 647 



Estorellyte.] 



Mic. In a fine-grained, transparent matrix, consisting mostly of interlocking 

 fresh, glassy feldspars, with a very few quartzes, some biotite, apatite and calcite, 

 frequent cubic grains apparently of magnetite, and some shreds of hornblende and 

 fragments of augite, lie large crystals of other feldspar, of augite and of hornblende. 

 The feldspar crystals are much twinned on the Carlsbad, the albite and the 

 pericline plans, and are rarely also surrounded by a zone, generally narrow, of a 

 later growth. These feldspars are not fresh, but are dimmed by kaolinic disin- 

 tegration. The zonal additions are dimmed also by the same disintegration, but far 

 less than the central areas. They are worthy of very careful investigation. They 

 are very interesting in their forms and especially in their twinning. But as the 

 object of this examination is to throw light on the origin and relations of the rock 

 masses principally, and as that purpose can be secured without further description, 

 these are left for future research. Further, Dr. Grant, in one of the annual reports, 

 has reported on this rock and has investigated the feldspar as well as the augite 

 (Annual Report, xxi, page 43). From chemical analysis he determined it to be 

 anorthoclase. 



In this rock the feldspars are larger than seen elsewhere, and, with the augites, 

 are better preserved. Many of the augites are uralitized (i. e., converted to horn- 

 blende) and many of them are mere fragments, comparable with the fragments 



already noted in the green schists (Nos. 1058, 1059 and 1060). 

 It is but rare that the form of a full crystal can be found in 

 the three slides examined, while fragments of all dimensions 

 are scattered everywhere, and in one slide it has furnished 

 much hornblendic dust, which dims the matrix. 



The feldspars in No. 1061 are rather compounded than 

 twinned, and the rock might be described as glomero- 

 40. porphyritic. The outline and compound structure of one 



of these crystals are shown in figure 40, magnified about twenty-five diameters. 

 Three sections. 



Aije. Archean (cutting Upper Keewatin). 



Remark. The name esterelhjtc was given by Michel Levy in 1897 to a rock 

 almost identical which occurs as laccoliths in the Mesozoic in "southern France. 

 ( .W>ii>iri'K/<r In /><>rphi//r Mru t/rT tisfn-/'/, Bulletin 57, Carte geologique de France, 1897.) 

 According to the determination of Michel Levy, based on chemical as well as 

 optical examination, the feldspar of esterellyte is of at least two kinds. The central 

 cores are of l<tl>ru<luril<' and the zonal increments are of tnnlc^iitc, which is in accord 

 with the optical specification of the feldspars in the porphyry of Kekequabic lake. 

 (Compare No. 1094.) N. H. w. 



