LITHOLOGY. l8l 



of it has been broken away. It is, however, as fine a specimen as could 

 be desired, to show a combination of two systems of twinning. When 

 revolved between crossed Nicol prisms, this section reaches its maximum 

 of darkness when a side is parallel to the plane of vibration of the light. 

 The ortho-diagonal therefore lies in the section; and if the section is 

 prismatic, then one comi^osition plane is parallel to the clino-diagonal, 

 and the other composition plane is the clino-dome, and we have a double 

 twinning, which is not uncommonly found in crystals which have devel- 

 oped in cavities, but which is very rare in ingrown crystals. The con- 

 trasts of color in the section are not very great, for, in a prismatic section 

 cut approximately parallel to the ortho-diagonal, neither kind of revolu- 

 tion throws the axes of elasticity far away from one another. Baveno 

 twins have been found — first, by E. Weiss — in the sanidin of trachytes 

 and modern eruptive rocks, but I am not aware of their having been ob- 

 served in quartz porphyries, though they have been frequently observed, 

 first by Rose, in granite.* 



This porphyry contains odd little concretionary masses of hornblende 

 and chlorite, which fill spaces that were perhaps originally empty. The 

 mineral grains on the outside are stained yellow by decomposition, and 

 thus the bright green minerals within are surrounded by a yellow wreath. 

 Crystals of orthoclase, which are pure within and impure without, are 

 also found here. The porphyries of this place are stated by Prof. Hitch- 

 cock to be plainly eruptive. 



A gray quartz porphyry from New Zealand contains so many large 

 crystals of quartz and feldspar, that the porphyritic character is almost 

 obliterated; and from this rock to granite is not a wide step. 



A gray quartz porphyry from Waterville possesses a very fine felsitic 

 base; and the sections of many of the quartz grains are polygons, which 

 indicate the presence of dehexahedrons of quartz that can often be ma- 

 croscopically seen. As is usual in such porphyries, these crystals have 

 rounded edges. 



This porphyry is a favorable one in which to observe the innumerable 

 little cavities which are characteristically present in the quartz of all 

 porphyries. When highly magnified, in these cavities a fluid and a bubble 

 are almost always seen, and, moreover, a little colorless cube is often 



^- Rosetibtisch Physiographie der iiiassigm Gesteene,'^. 12. 



