382 



affected the distribution of the other constituents. Mica, for example, 

 is less abundant in the andalusite than it is in the other portions 

 of the rock. The spots in certain spotted-schists are, as Mr. WARD 

 pointed out, merely undeveloped (that is, undeveloped so far as form 

 is concerned) crystals of andalusite. Such spots must not be confused 

 with those which are due to a local heaping up of pigment. 

 They belong to a totally different category and occupy a different 

 position in the zone of metamorphic action. Andalusite is a silicate 

 of alumina (A1 2 3 , Si0 2 ). It occurs in argillaceous sediments near 

 granitic masses and also in crystalline schists. The andalusite-schists 

 of Aberdeenshire, described by Mr. HORNE (I) , are similar to many 

 contact-rocks. Unless we assume the existence of an underlying mass 

 of granite, which is nowhere exposed, or the former existence of an 

 overlying mass, we must agree with Mr. HORNE that these rocks are 

 the result of regional metamorphism. 



Chiastolite. This mineral is closely related to andalusite. It 

 possesses the same composition and the same prismatic angle. It is 

 developed in long prisms. Cross-sections of these prisms are nearly 

 rectangular. Oblique sections are more or less lozenge shaped. The 

 peculiarity ot chiastolite lies in the fact that the central portion of 

 the crystal is usually occupied by inclusions of dark substance 

 corresponding to the matrix of the slate or schist in which the 

 chiastolite has been developed. The outlines of the central patch in 

 any section correspond with those of the crystal itself. Lines of dark 

 inclusions run out from the angles of the central patch to those of 

 the crystal itself (made te'tragramme of HAUY). Sometimes the angles 

 of the prisms are occupied by four dark rhombs corresponding in 

 form with those of the central core (made pentarhombique of 

 HAUY). Chiastolite occurs in clay-slates near granite-masses. Some- 

 times chiastolite and andalusite occur in the same rock. As a rule, 

 however, the chiastolite is found in the outer zone of metamorphic 

 action, whereas andalusite is more characteristic of the inner zone 

 and is often found in the rock which is in actual contact with the 

 granite, as in the Lake District, Wicklow and Cornwall. 



Staurolite. This mineral is also orthorhombic. The common 

 forms are (110), (001), (010) and (101). Longitudinal sections are 

 rectangular ; cross sections often nearly hexagonal. The prismatic 

 angle is about 130. The mineral often occurs in the form of 

 penetration twins. When the two individuals form a rectangular 

 cross the twin plane is a brachydome (032) ; when they cut each 

 other at an angle of about 60 the twin plane is (232). Refraction 

 and double-refraction are both strong. The vertical axis is the acute 

 bisectrix and the double-refraction is positive. The pleochroism is 

 given by ROSENBUSCH as follows : a and @ yellowish red often with 

 a trace of green ; 7 hyacinth-red to blood-red. Staurolite is more 

 complex in composition than the other aluminous silicates which are 



(1) M.M., Vol. VI. (1884), p. 98. 



