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 Pl. 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. CorumsBitE [Fe (Cb, Ta)? O°]. 
This comparatively rare mineral has been found in the granitic veins 
of Acworth. The crystals there found possess an individual character 
VOL. Iv. 16 
