50 MINERALOGY AND LITHOLOGY. 



Hanover; jasper, also, at the same places; quartz, penetrated through 

 and through with tourmaline needles, at Sullivan and on Moose moun- 

 tain, besides numberless occurrences of local interest. 



The white translucent variety of quartz is found in large beds at Al- 

 stead, Hancock, Bedford, Amherst, and Lyndeborough ; and Prof. Hitch- 

 cock has traced a system of veins of it which extend for more than fifty 

 miles through the gneissoid rocks. At times, a kind of quartz filled with 

 cavities, making what is called buhrstone, is found in veins. This kind 

 of quartz occurs at Littleton, 



At Grafton a very large, interesting crystal has been found, which is 

 in the collection of the survey. It consists of a huge crystal a foot long 

 and eight inches in diameter, but which is entirely made of little crystals 

 with their planes all parallel to the planes of the large crystal. This 

 complex form of a quartz crystal is not uncommon, but such large, fine 

 specimens are rarely seen. 



The microscopic characters of quartz are so fully illustrated in the 

 lithological part of this report as to need little explanation here. It may 

 be briefly stated, however, that basal sections of quartz differ from other 

 hexagonal minerals, in that a beam of light passing through them par- 

 allel to the vertical axis is rotated to a certain degree; and hence quartz 

 between crossed Nicols exhibits the phenomenon of circular polariza- 

 tion. The amount of rotation of the light depends upon the thickness 

 of the plate ; and also the different colors of the spectrum are rotated to 

 a different degree. Hence, if a plate of quartz thus cut, and of some 

 thickness, is inserted between crossed Nicol prisms, the plate will not be 

 dark, as in the case of ordinary hexagonal minerals ; and neither will the 

 plate be dark, whatever be the position of the Nicols with reference to 

 one another ; but as the upper Nicol prism is revolved it will meet the 

 different colors of the spectrum in succession, and the amount that it 

 must be turned to intercept all the colors from red to violet will depend 

 on the thickness of the plate ; and whether the polarizer must be turned 

 to the right or the left, in order to intercept the colors in the order of 

 their arrangement, beginning at the red end of the spectrum and pro- 

 ceeding towards the violet, determines whether the crystal is right- or 

 left-handed. If, now, the section of quartz be cut very thin, it is plain 

 that the revolution of the light may be so small as to be imperceptible. 



