l80 MINERALOGY AND LITHOLOGY. 



tides 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 PI. lo. It lies on the edge of a section, and so half 



