180 MINERALOGY AND LITHOLOGY. 
ticles of the other minerals, which had formed or were forming in the 
rock. At first the particles were small, and were included in large num- 
bers, but afterwards they were larger and more sparingly distributed. 
That this line between the pure and impure parts of the crystal is indic- 
ative of some important change, is shown by the circumstance that this 
line also separates two parts of the crystal in which the optical constants 
are differently orientated. For this reason the outer and inner zones 
appear differently colored in polarized light; and the difference is more 
plainly marked by revolving the section till one part is dark, when it will 
be seen that the section must be turned a certain number of degrees 
from this point in order to induce the maximum of darkness in the other 
part. This indicates that the directions in which the planes of elasticity 
cut this section make an angle at the point which separates the pure and 
impure portions of the crystal. These directions are indicated in the 
figure; and in the case of the crystal represented, the difference between 
them is five degrees. All the porphyritic crystals of orthoclase in this 
rock possess these noticeable peculiarities, which are interesting as giv- 
ing evidence of the existence of stages of development. Indeed, the 
development of porphyries must depend more or less on the existence of 
stages, else the crystallization of large grains would have continued till 
the end. 
‘The presence of augite in large and small grains is noticeable in this 
porphyry. ‘It is not so very abundant as it is in the augitic porphyries 
about Leipzig, where it exists in such quantity as to relate the rocks 
which also contain plagioclase to diabase. But it is in this case, and in 
some others, a very characteristic feature of the rock. 
Sections of a gray—almost black—quartz porphyry from Groveton 
offer other microscopic peculiarities. The porphyritic crystals are mostly 
of orthoclase, some of which are subjected to exceptional methods of 
twinning. Compound crystals of orthoclase, when ingrown in the rocks, 
are usually Carlsbad twins, those of other kinds being very rare. In this 
Groveton porphyry most of the crystals are Carlsbad twins; but some of 
the crystals are also twinned according to a different system, the compo- 
sition plane being the clino-dome, and the twins consequently Baveno 
twins. The crystal which is most favorably cut to show this is repre- 
sented in Fig. 4 on Pl. 10. It lies on the edge of a section, and so half 
