PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 



Tuff.] 



643 



while it is structurally and petrographically a part of the same series as Nos. 1047 and 

 1048. Another mineralogic link connecting this rock (No. 1049) with No. 1043, lies in 

 the nature of the hornblende which has the characteristic which optically distinguishes 

 pargasite from all the other hornblendes, viz. : it has the axis g in the acute optic angle. 



N. H. W. 



No. 1050. TUFF. (Modified.; 



From the same bluff, near the top. 



R?f. Annual Report, xv, pages 304, 365, 395; Annual Report, xvii, pages 190, 206. 



Meg. Grayish green, rather compact. 



Mir. Essentially the same rock as the foregoing from this bluff. The fine 

 matrix is not so abundant and the porphyritic aspect of the hornblendes is not so 

 marked. They are also smaller, bringing the rock into greater homogeniety, still 

 characteristically the same rock with the addition of a few small grains of quartz 

 (or cordierite?) and a greater proportionate amount of hornblende. Two sections. 



Age. Archean. N. H. w. 



No. 1051. TUFF. (Modified, granitic.) 



West shore of Kekequabic lake; sec. 3, T. 64-7. 

 Ref. Annual Report, xv, pages 364, 395. 



M<-</. Aspect of an orthoclastic rock; in the field was styled orthoclase gabbro. 

 from the top of the bluff which at the bottom is a pebbly green schist. 



.1//V. This rock seems to unite the characters of Nos. 1044, 1045 and 104(5 with 

 those of Nos. 1047 and 10oO. The matrix material, however, is coarser, composed of 

 a biaxial mineral of low refraction and low double refraction, having n s in the small 

 optic angle without cleavage and without twinning, which embraces all the other 

 minerals poikilitically. The most of the rest of the rock is hornblende in small 

 crystals and spicules which pierce the translucent mineral mentioned in all directions 

 and with great freedom. This poikilitic translucent mineral, which in the hand 

 sample is sometimes reddish, gives a fine granitic aspect to the rock. 



In one large grain there appear to be two growths, 

 one (the later) surrounding and penetrating the other, 

 the two extinguishing in common in a somewhat spheru- 

 litic succession, as shown by the accompanying figure. 

 The later growth has slightly higher refraction, as indi- 

 cated by the Becke line, and higher double refraction. But 

 these differences may be due to different positions of 

 the same mineral. The sketch herewith (figure 38) outlines 

 roughly the manner in which the older feldspar(?) grains 

 are related to the later growths. The later growths cross 

 STR?VTUR THE L FORM L ER the older in. fissures and in irregular lamella?. They are 

 *^! ERYOBLIQUE : THE of markedly different color, of double refraction, being 



FIG. :. INTERGROWN ORTHO- 



