C. H. Smyth, Jr. — Dikes of Alnoite at Manheim, iT. Y. 291 



lar, showing an abundance of mica. The structure in these 

 central portions has been considerably modified by shearing, 

 the rock being crushed, slickensided and much altered. Unfor- 

 tunately, this has rendered it difficult to procure satisfactory 

 material for study from the coarser parts of the rock. 



A notable feature of the dike is the presence in it of long, 

 narrow " horses " of the country rock. One of these is fifteen 

 or twenty feet long, about one foot wide, and with tapering 

 ends. These horses affect the dike just as the walls do, render- 

 ing the rock fine-grained. The rock of the walls and horses 

 shows very scanty signs of contact metamorphism, consisting 

 of some slight changes of color. 



The color of the fresh dike rock is very dark-gray to black ; 

 but there is often a tinge of green from alteration and this is 

 sometimes pronounced. Further decomposition breaks the 

 rock down ta a sandy clay with scattered flakes of mica. 

 Under the microscope, this clay is seen to consist chiefly of 

 finely-divided mica, with minor magnetite and apatite, the 

 other minerals having disappeared. That such complete altera- 

 tion and disintegration take place readily is shown by the 

 occurrence of the dikes in deeply excavated trenches. It is 

 also perfectly displayed on the opposite side of the stream, 

 where what is doubtless the continuation of the large dike is 

 shown. It is a vertical band of the clay above described, cut- 

 ting sharply through the Calciferous, and exposed for a height 

 of about twenty feet. The wall rock is perfectly fresh, but 

 the dike is so thoroughly decomposed that only the scales of 

 mica and an occasional lump of alnoite the size of a walnut 

 show the true origin of the material. 



Sections of the rock of the small dike agree perfectly with 

 the original dike, and need no further consideration. The large 

 dike, however, shows decided differences, with one exception, of 

 such a nature as would be expected from its coarser structure. 



Sections of specimens taken from near the margins of the 

 dike, or close to the included horses, have a very fine ground- 

 mass, in some cases looking almost like an altered glass. In 

 this are scattered phenocrysts of deep-brown mica and altered 

 olivin. The latter mineral, in these parts of the dike, is usually 

 represented by a semi-opaque, white substance, through which 

 is disseminated a black dust, doubtless magnetite. This method 

 of alteration of olivine seems to be dependent upon proximity 

 to the wall rock, as it does not appear in the central portions 

 of the dike. 



Sections from the latter approach a granular structure, and 

 consist chiefly of mica and altered olivine. The structure is 

 not that of a granite-granular or hypidiomorphic rock, as the 

 two chief minerals are quite commonly bounded by their true 

 crystal form. The ,mica has a deep reddish-brown color, is 



