318 



Scientific Geology. 



4. Quartz and Mica. This differs from mica slate, only in the 

 predominance of the quartz. Usually this mineral is greatly in ex- 

 cess : but occasionally the quantity of mica increases so much, that it 

 is impossible to say of particular specimens to which rock they ought 

 to be referred. In such cases I have taken into consideration the 

 character of the surrounding region. If mica slate predominates, and 

 there be not an actual passage into decided quartz rock, I have thought 

 it useless to describe the rock as quartz rock, even if for a considera- 

 ble extent the quartz predominates. Such cases as this are common 

 in the mica slate range extending from the mouth of the Merrimack to 

 Connecticut. And on the other hand, if quartz rock predominates, an 

 occasional excess of mica in some of its strata has not prevented me 

 from considering the whole as quartz rock. 



The mica in this variety is arranged in a parallel position, and it 

 produces a schistose structure ; though sometimes the laminae are so 

 thick that they ought rather to be regarded as strata. In other in- 

 stances, the schistose layers are extremely tortuous and very distinct 

 from the stratification. I have observed this circumstance only in 

 Berkshire County, as in Lee. (Nos. 572, to 580 ; and 587 to 592.) 



5. Quartz and Talc. Some of the talcose slate in Hawley, Plain- 

 field, &c, occasionally becomes a slaty arenaceous quartz, with seams 

 of greenish talc. (No 581 to 583.) Its colour is white, and this rock, 

 seen at a distance in Hawley and Rowe, resembles gneiss. It is ob- 

 viously a member of the talcose slate formation ; and it may be ques- 

 tionable whether it ought not rather to described in connection with 

 that formation. 



6. Quartz and Hornblende. Instead of talc, the white arenaceous 

 quartz described under the last variety, sometimes contains numerous 

 distinct crystals of black hornblende. (Nos. 584, 585.) It forms a 

 beautiful rock, and if it would admit of a polish, might be employed 

 for ornamental purposes. It is less abundant than the preceding va- 

 riety. In the gneiss formation, there is a variety in which greenish 

 hyaline quartz contains flattened imperfect crystals of actynolite. 

 (No. 584.) 



7. Quartz and Argillaceous Slate. I have observed this only in 

 Bernardston, in connexion with the encrinal limestone. (No. 601,) 

 The quartz is white and blue, and exhibits a brecciated structure. It 

 was probably coloured by the slate ; but very few fragments or layers 

 of slate are now visible. 



8. Quartzose Breccia. This consists of angular fragments of gran- 



