238 



W. S. BAYLEY SYENITES FROM NEW ENGLAND. 



other components of (lie rock lie, and sometimes filling what were appar- 

 ently cracks in the rock mass (figure 2, plate 7). All these grains polarize 

 with bright colors, and all arc clear and perfectly transparent. They are 

 all of about the same size, none ever have crystallographic outlines, and 

 all are younger than the large crystals of albite that have been mentioned 

 st> frequently. In rare cases this mosaic itself is imbedded in a finer 

 mosaic of the same character, except that it is saturated with cancrinite. 

 The structure produced by the imbedding of the larger components of the 

 rock in this fine grained mosaic is strongly suggestive of the mortar 

 structure of Tornebohm, which is regarded by this author as a certain 

 indication that the rock exhibiting it has been subjected to pressure and 

 shearing. 



Two feldspars are distinctly observable in the mosaic, and a third one 

 may exist. The two undoubtedly present are so much alike in appearance 

 that it is difficult in many instances to determine the nature of a par- 

 ticular grain. The number of untwinned grains however indicates the 

 presence of an orthoclase, while t lie number of grains with straight narrow 

 twinning lamellae points to the existence of a plagioclase. Another feld- 

 spar almost surely present is microcline. It is in slightly larger pieces 

 than the other two. and is well marked by the double twinning. It is 

 impossible to speak more positively as to the nature of these feldspars, as 

 cleavage cracks are not common, crystallographic outlines are never 

 present, and tin 1 twinning lamella; are bowed and bent to such an extent 

 that readings of extinction angles are not decisive. 



In separation by the Thoulet solution two lots of feldspar fell when the 

 density of the liquid was 2.022 and 2.56 respectively. That which fell 

 at 2.G22 consists of grains usually striated in a single direction and of 

 others in which no striations arc noticeable. The latter extinguish at 

 19° from the cleavage, and show between crossed nicols the bar of an 

 axial figure. Their analysis, made by Mr. W. H. Melville, of the United 

 States Geological Survey, is that of a very pure albite (I) : 



