Copper-Tin Veins of tlie Akénobé District. 19 



of special attention that the main stock has undergone intense 

 dynamic processes, resulting, in places, in schistose diorite, mylonite, 

 <3tc., wliile the offshoots show frequently only a slight sign of 

 dynamo-metamorphism. The offshoots are often altered to green- 

 stones, particularly near the veins ; this alteration is believed to 

 be due to the hydrothermal processes. 



(D) Dyke Rocks. 



Numerous small dykes, ranging in thickness from a fraction 

 of one meter to several meters, are found in the vein district now 

 under consideration. Those dykes are often decomposed to gray 

 or whitish rocks with an earthy texture. As can be discerm.^d on. 

 microscopic study, they arc potrologically of diverse characters, the 

 important types being andésites, felsites, porphyries, porphyrites, 

 and diabases. 



HornUench-Hijpsrsthene Andésite. In the abandoned adit of 

 Ishikané, in the Daijukô area, where the eastern continuation of 

 the Daikoku vein was worked, two dykes are found across tho 

 vein. They strike N. E. and dip very steeply towards N. W. 

 The one nearer to the entrance of the adit has a thickness of 3 

 meters or more, the other of only a fraction of one meter. Tho 

 rock is dark coloured and compact. Wlien decomposed, phenocrysts 

 of feldspar and hornblende are clearly recognizable with the unaided 

 eye, the hornblende occasionally attaining a length of 5 mm. or 

 more, though commonly nmch shorter. 



Under the microscope, the groundmass shows a trachytic 

 structure consisting of innumerable plagioclase laths in parallel or 

 lluidal arrangement, mingled with abundant long prismatic microlites 

 of rliombic pyroxene and fine crystals of magnetite. No glass-base 

 is recognizable; it has probably been dcvitrified into indistinct 



