PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 109 



Tuff.] 



are sometimes brown and sometimes light green. Some epidote is distinguishable by 

 its high refractive power; and large areas are separable from the rest by reason of 

 a greater darkness, or by a curly, minute, somewhat crescentic structure which 

 embraces the translucent and greenish grains, the crescentic areas themselves 

 being most translucent. 



One section examined. 



Age. Cabotian. N. H. w. 



No. 8B. TUFF (?) 



Duluth. At the lake shore near the base of Minnesota point (the spot is now hid by the growth of the city). 

 Rcf. Annual Report, ix, pages 12, 13. Annual Report, x, page 140. American Association for the 

 Advancement of Science, vol. xxx, page 164, 



Meg. This is a light colored rock with disseminated red feldspar crystals, 

 having the aspect of a porphyry. A light greenish yellow mineral, probably epidote, 

 more or less pervades the rock, this being the principal cause of the light colored 

 aspect. The rock effervesces freely with dilute hydrochloric acid. The feldspar 

 crystals are polysynthetically twinned, and the rock has in places a roughly amygdal- 

 oidal structure. This structure, however, is not sufficiently prevalent and character- 

 istic to indicate that the rock en masse was ever in the form of a surface flow. The 

 matrix of the feldspar crystals is confused and made up of various minerals that 

 result frequently from the alteration of the minerals of original basic rocks. 



NIC. The rock is much altered. The feldspar is charged with ferruginous 

 impurities. A series of ten statistical measurements of the extinction angle on 

 opposite sides of the twinning line gave the following figures: 9, 9; 19, 4; 9, 

 1H; 10, 8; 114, 6; 18, 5; 5, 15; 10, 1; 10, 18; 8, 20. 



If the first measurement (9, 9) be taken as the maximum equal extinction, 

 according to the spherical projections of Michel Levy (Det. des Feldspaths, plate 1) 

 the feldspar is <ill>it<', but owing to its greatly decayed condition this result is not 

 beyond doubt, while the effervescence, indicating calcite, seems to require the 

 presence of a ready source for considerable lime. 



T&o pyroxene element can be detected, nor indication of its earlier existence. 

 The angular spaces which are sometimes surrounded by the feldspars may have been 

 occupied originally by a pyroxene which has now been changed, but at the same time 

 there may have been a variety of minerals so included between the feldspars, or even 

 tufa pulp, or a glass, the time elapsed having been sufficient to change either of 

 these into the matrix that now exists. 



(Jitartz is common, but not as a pegmatitic intergrowth in the feldspar. It is 

 sometimes in angular grains, isolated and irregular, and it sometimes embraces 

 portions of the surrounding chlorite and other minerals. It is also in nests resulting 

 from secretion from the rock, and then presents a multiple or aggregate polarization. 



