76 MINERALOGY AND LITHOLOGY. 



quartz. Some of these little crystals exist in the sienite at Sandwich, 

 and are there very perfect in form. 



49. Vesuvianite (Idocrase) [Cas AI4 Sir Oas]. 



Large and fine crystals of vesuvianite have been found at Amherst, 

 and also at Warren. These crystals occur on the surfaces of contact of 

 the limestones and siliceous schists that are there associated. One of 

 the crystals from Amherst is represented in Fig. 2 on PI. 3. It is taken 

 from the second edition of Dana's Mineralogy. 



50. Epidote [W Ca' (AP, Fe')^ Si« C]. 



We have epidote both in isolated crystals and in our rocks. It is 

 found at Lisbon in light yellow acicular crystals, and in larger, finer forms. 

 Very pretty twin crystals, and also a massive variety, are found there. It 

 occurs at Warren, associated with quartz and j^yrites. It fills a vein in 

 Jackson, from which immense crystals have been taken, some of which 

 were eight inches in diameter and of a fine green color (Jackson). Smaller 

 but better crystals, and also twins, are more common. It is found at 

 Bedford, Gilmanton, Hanover, Portsmouth (radiated acicular crystals in 

 hornblende), Exeter (very beautiful groups of radiating crystals), and 

 Benton (in boulders). 



As a rock constituent, epidote is preeminently characteristic as a 

 decomposition product in certain basic rocks. It is found in seams and 

 cracks in the metamorphic diorites that are so common in the Connecti- 

 cut valley, and is an almost constant microscopic ingredient of the rocks 

 themselves. The same is true of some" of our eruptive rocks. In micro- 

 scopic sections it is trichroic ; but the colors are often so faint as to 

 render the recognition of this difficult. It may be said, however, that it 

 is usually yellow in color, and, when revolved between crossed Nicols, the 

 colors obtained are perhaps more brilliant than those shown by any other 

 mineral. 



Where the rocks contain the most basic feldspars, and where this feld- 

 spar is dull by decomposition, there epidote is most often found. In our 

 light green feldspathic diorites, epidote seems to be the mineral most 

 commonly found in and about the triclinic feldspar as a product of its 

 decay. At times, this process of alteration has proceeded so far that 



