Il8 MINERALOGY AND LITHOLOGY. 



the latter species has often a slight angle between its two optic axes. 

 Chlorite forms large aggregations which might be termed rocks, and 

 besides its occurrence in what would be termed mineral specimens, it is 

 an essential constituent of chlorite schist, diabase, &c. The particular 

 species of a chlorite which exists in a chlorite schist is often hard to 

 determine, and the more so because more than one species are liable 

 to be present together. For example : in some schists there are basal sec- 

 tions of chlorite which remain dark between crossed Nicol prisms during 

 a whole revolution (hexagonal), while others, in position likewise parallel 

 to the cleavage or base, do not remain dark, and are therefore biaxial. 

 The name viridite is reserved for those green chloritic products which 

 can be referred to no species, which cannot be isolated for examination, 

 and in reference to which it only is known that their appearance and 

 association indicate them to be chlorites. The chlorites are usually 

 dichroic, with the exception of the viridite which acts at times like an 

 amorphous substance. 



Besides its occurrence in the rocks, chlorite is found often in radiated 

 and foliated aggregations, in cavities and clefts. The process of hydra- 

 tion and alteration of basic rocks usually gives rise to the formation of 

 more or less chlorite. The following species have been determined in 

 our state. 



77. RiPIDOLITE (MONOCLINIC) [Mg' AP Si^' O" + 4H^0]. 



This is probably the chlorite that is most abundant in the chlorite 

 schists. It is also found in the beds of talc and talc schist, and it occurs 

 in well formed crystals at Orford. It is bright green in its color, but 

 strongly dichroic, being green in the direction of the vertical axis, while 

 it is brown or yellow in a transverse direction. This is best observed in 

 thin sections under the microscope. In these sections, as a rule, no 

 crystalline form can be noted, but all the sections are double-refracting. 

 This chlorite occurs in rocks that do not contain large percentages of 

 iron, and do contain considerable magnesia. It may be an original 

 product, and it may be the result of the alteration of other minerals. 

 Between crossed Nicols it gives brilliant interference colors, and is 

 usually more or less impure from the presence of magnetite. 



