232 MINERALOGY AND LITHOLOGY. 



mobile or homogeneous mass. Many of these schists contain numerous 

 biotite crystals, the bright cleavage faces of which are all parallel to the 

 plane of stratification, and they dot the dark rock with black shining 

 spots. Such a specimen comes from Berlin station. The crystals of bio- 

 tite are sometimes very much lengthened in one direction, and then 

 the rock is streaked with bright black lines. A rock of this kind occurs 

 at Dummer. Some specimens are almost all hornblende, and others are 

 half quartz. Some specimens are almost massive, and others are very 

 schistose. Some are almost black, others are light green, and when thin 

 sections are cut from this latter class they are found to contain much epi- 

 dote and considerable chlorite, by which the lightness of the color is 

 produced. The hornblende may vary in color and give analogous results. 

 Under the microscope all these varieties present the same fibrous struc- 

 ture, and no peculiarities that have not been mentioned. Hornblende 

 schist is not confined to the valley of the Connecticut, but all the varie- 

 ties that occur elsewhere are there to be observed. 



Actinolite Schist. This rock contains the Hght-colored fibrous actino- 

 lite in place of the dark hornblende. It is therefore very light green ; 

 and in thin sections the hornblende is almost colorless. In an actinolite 

 schist from Pittsburg, quartz, and some plagioclase, biotite, and chlorite, 

 are included between the needles of actinolite; but the numerous crys- 

 tals and needles of rutile that it contains form a striking feature. This 

 mineral is present both in the quartz and in the actinolite, but its condi- 

 tion is different in the two minerals. In the quartz, it exists in extremely 

 fine, long black needles, which run in all directions, sometimes in straight 

 lines and sometimes in curves. In the actinolite, on the contrary, it 

 exists in much larger, deep yellow crystals, which are in part very well 

 crystallized, and which often show the peculiar geniculations to which 

 the species is subjected by its mode of twinning. A thin section of this 

 rock is represented in Pig. i on PI. 4. 



CHLORITE SCHIST. 



This rock is related to the argillitic mica schist that has been described, 

 and is often associated with it. Intermediate varieties between the two 

 rocks are abundant; and they differ only by the substitution of chlorite 



