MINERALOGY. 121 



cated by the analysis given above. As observed in thin sections, this 

 chlorite, which is formed by the decomposition of basaltic, ferruginous 

 minerals, often refracts the light so as to give the brightest colors between 

 crossed Nicols, and often it is seen as an aggregate of fine scales; and, 

 again, a green, ferruginous chloritic substance is often found in such 

 rocks, which acts on the light like a perfectly amorphous body. It is 

 black in every position between crossed Nicols, as is a glassy substance. 

 This at first deceived observers, but it is now known that this substance 

 is chloritic in its nature. For such products as this last, and for other 

 products of this nature, the compositions of which are only approximately 

 understood, and which can be with justice referred to no mineral species, 

 the name viridite is applied. 



In our diabase rocks, chlorite sometimes takes very pretty microscopic 

 forms when its surroundings allow it to crystallize freely. For example: 

 in PI. 8, Fig. 5, is a representation of a section of diabase in which is 

 seen a kind of cavity filled with calcite, and in the calcite are numerous 

 little spherical concretions of chlorite. They are light green in color, 

 and their radiated structure is not very evident. If, now, we put them 

 between crossed Nicols, all their structure is developed, and they appear 

 as radial discs traversed by a black cross, and look very pretty. This is 

 what is illustrated in the figure, which will be readily understood. If the 

 concretions are made of needles or plates of an hexagonal mineral, all 

 those crystals which coincide with the plane of either Nicol prism will 

 be black, while all others will be colored. As, now, in these concretions, 

 crystals radiate out in all directions, each concretion must be crossed by 

 two bars coinciding with the planes of the Nicols. The planes of the 

 Nicols are indicated in the figure by the spider lines. This figure is 

 drawn from a section of the diabase of Dixville. The centres of some 

 of the concretions are filled with calcite. The other minerals in the 

 section are a triclinic feldspar, recognized by its striations ; augite, col- 

 ored blue and yellow by interference of the rays; brown biotite; and 

 the light calcite, with its characteristic cleavage. 



81. CoLUMBiTE [Fe (Cb, Ta)-^ 0«]. 



This comparatively rare mineral has been found in the granitic veins 

 of Acworth. The crystals there found possess an individual character 



VOL. IV. 16 



