ON COEDIERITE AS CONTACT MINERAL. 62 1 



mica. The peculiar pseudomorph from Tamba already mentioned be- 

 longs no doubt to this category. In the specimens from the \Vatarase- 

 gawa region, we can trace under the microscope the progress of the grad- 

 ual transmutation from the transparent fresh mineral to its final stacre. 



The alteration first sets in from the irregular cracks traversing 

 the crystal, somewhat like the formation of Serpentine from Olivine. 

 In this first stage all that we can observe is, that there is formed along 

 these cracks a greenish or white impellucid material in the form of a 

 double layer (Fig. 10). This layer is found on close examination to 

 consist of crystalline aggregates of a very fine fibrous mineral, exhibit- 

 ing under crossed Niçois, an irregular aggregate-polarization. In 

 most cases, however, we observe that the alteration proceeds along a 

 series of very fine linear fissures, which are differently arranged in the 

 different portions of the crystal, analogous to the arrangement of the 

 enclosures already described. Figure 10 shows these fissures in a sec- 

 tion parallel to the vertical axis, the irregular cracks, from which the 

 alteration started, running in a transverse direction, while the linear 

 fissures run at right angles to it. 



The alteration gradually makes its progress, until there is 

 left no trace of the original substance, and the final product is an 

 impellucid crystalline substance which usually bears a greenish tinge. 

 It is usually mixed up with some greenish coloured mica, which is 

 sometimes only very sparingly developed, or often fills up almost the 

 whole space. But it is in the specimens from Tamba that this mica 

 is most typically developed as the sole product of alteration. In its 

 outer aspect the altered product of the Watarase-gawa region generally 

 looks like Aspasiolite from Kragaroé, Finnland, while that of 

 Tamba resembles Ghloraghyllite from Haddam, Connecticut,- U. 8. A. 



The pseudomorph from Tamba is of a greenish colour, imbedd- 



