MINERALOGY. 7I 



quite imperfect, indicating a poor development of the prismatic planes, 

 while the sections parallel to the vertical axis show beautiful six-sided 

 forms, being combinations of the edges of a prism and dome. The ap- 

 pearance of the olivine in the olivine diabase of Campton falls is repre- 

 sented in Fig. 4 on PI. 7. Beside these crystalline outlines, the micro- 

 scopic peculiarities of olivine are very characteristic. As a rule, its 

 cleavage is wholly irregular, though cases are not wanting in our rocks 

 where it gives evidence of a very perfect cleavage parallel to the plane 

 of the macropinnacoid. The crystals are dark between crossed Nicol 

 prisms when a prismatic edge or a cleavage, when evident, is parallel with 

 the plane of vibration of the light. The interference figures produced by 

 olivine, when revolved between crossed Nicol prisms, are very brilliant. 



The decomposition of olivine is very characteristic. As before stated, 

 it decomposes with great ease ; and almost always, in microscopic sections, 

 decomposition products of one kind or another are found about the fract- 

 ures or cleavages, which have admitted the reagents that act upon it. 

 In the olivine of our gabbrqs, alteration has not progressed far ; but in 

 the diabases the alteration is almost complete. For example : in the crys- 

 tals figured from Campton falls, the material about the cracks is a differ- 

 ent substance from the olivine, and is of a light yellow color. By its 

 action on the light, it is recognized as serpentine, the common product 

 that results from the hydration of olivine. This shades off into a greenish 

 yellow fibrous serpentine, while only the centres of the larger crystals are 

 still intact. All these different products are brought into the strongest 

 relief by the aid of polarized light. Many crystals have been observed 

 and described by Zirkel and others, that are entirely altered into serpen- 

 tine; and the well known crystals from Snarum, in Norway, afford excel- 

 lent macroscopic illustration of this change of olivine into serpentine, 

 which the microscope finds so common. 



47. Garnet [R3 R Sis O12]. 



R, in our species, standing for lime, protoxide of iron, and manganese ; 

 R for alumina and sesquioxide of iron. 



Garnet is a common mineral in our metamorphic rocks. It is found 

 usually in very perfect crystals, the forms being the dodecahedron and 

 the tetragonal trisoctahedron or trapezohedron, and, again, it is found in 



