122 MINERALOGY AND LITHOLOGY. 



from the large development of very acute octahedral planes, which pro- 

 duce long, pryamidal summits on the crystals. These crystals were first 

 described by Prof. C. U. Shepard, who made an excursion to Acworth 

 for minerals in 1830. 



Columbite is a very heavy mineral, but there are wide variations in the 

 gravities that have been taken upon specimens from various localities. 

 This variation has been shown by Marignac to be due to the varying 

 amount of tantalic acid, which increases the specific gravity in propor- 

 tion as it is present in greater amounts. Pure columbate of iron has 

 a gravity of 5.4, and pure tantalate of iron has a gravity of 8. Most 

 of the columbites that have been analyzed are isomorphous mixtures of 

 these two compounds, and have intermediate gravities. Marignac,* who 

 investigated this subject, examined the columbite from Acworth, and he 

 found that it had a specific gravity of 5.65, and, in accordance with the 

 rule deduced, it contained 15.8 per cent, of tantaUc acid. Columbite is 

 also found associated with beryls at Plymouth. 



83. Apatite. [Ca^ F O', (CI, Fl)]. 



This mineral is found abundantly in the vein of feldspar and quartz at 

 Westmoreland, which has been previously referred to as a locality for 

 molybdenite. The hexagonal crystals of apatite are there found abun- 

 dantly, and are often large. Blue and green apatite in very pretty crys- 

 tals is found at Grafton. Fine crystals occur in a bed of white lime- 

 stone at Piermont. Apatite is also found in Jackson. 



Besides its occurrence in these fine crystalline varieties, it is almost 

 universally spread about as a microscopic accessory constituent of the 

 rocks. Though not constant, there is no variety of rock in which it is 

 not sometimes found. When the crystals are not too minute it is easily 

 recognized in thin sections, because it is always well crystallized, and 

 consequently its sections are either hexagons or parallelograms. These 

 needles and prisms of apatite pierce through all the other minerals that 

 are common in rocks, and thus indicate that apatite was the first formed. 

 The crystals are usually quite long in proportion to their size, a charac- 

 ter that distinguishes them from nephelinite, which is almost the only 

 mineral with which they might be confounded, but which is not found 

 in our rocks. The sections of apatite are usually colorless, but when 



* Archiv des Sci. Physiques et Nat. Nouvelics, xxv, p. 24. 



