ON CORDIERITE AS CONTACT MINERAL. 329 



into mica is rather abrupt, there being no evident sign of gradual 



passage. We are rather inclined to think that the formation of mica 

 may directly follow as a result of alteration. For, in some cases, we 

 rind, that the tissures of the crystal are lined abruptly with the 

 mica flakes, as if a thin clear cleavage-piece of this mineral were insert- 

 ed between the fresh fissures. In such eases, which we could observe 

 very distinctly in a specimen from the Watarase-gawa, the mica 

 begins either to be formed very regularly disposed at right angles ti- 

 the vertical axis, or from the sides of the crystal, sometimes parallel, 

 sometimes very irregularly disposed, to the outer wall. 



It would be of interest to know the chemical alteration which the 

 altered Cordierite has undergone. A sample of the Tamba pseudo- 

 morph free from admixture, was analyzed, with care, by Air. Tamura 

 of the laboratory of the Geological Survey. The result was as 

 follows : — 



Si0 2 4092% 



AW, 31-06 



FeO 7-99 



CaO trace. 



MgO 6-71 



KJ) 8-60 



Na 2 -72 



Loss on ignition 3*22 



99-22 G = 2-77. 



By comparing this result with that already given, we see there 

 is a considerable addition of alkalies (potash) and of water, while the 

 amount of silica is less. This pseudomorph, therefore, belongs to 

 that class of the alteration-products of Cordierite, which has taken up 

 not only water but also potash, and is essentially analogous to that 



