XI 8 N. YAMASAKI. 



formation, and rhyolite and propylite (apo-andesite) make their appear- 

 ance through it. The principal rocks of the tertiary strata are shale 

 and coarse, green, tuffogenic sediment. The former, which is generally 

 the older, is well developed in the northern part of the district, and 

 the latter in the southern part. Sometimes, however, one occurs 

 interstratified with the other in thin layers. The direction of strike 

 runs S— W to N-S, with slight deviation in some places. A special 

 geological feature of the sedirnen taries can be seen in connection 

 with a large mass of rhyolite in the centre of the district, where the 

 igneous rock covers the tertiary shale at the foot of the Djizo-pass, 

 while on the western part of the range, it is covered by the shale. 

 Besides the large mass of rhyolite there are also small dykes of it, 

 some good exposures of which are easily recognisable in the railway- 

 cutting along the Chikuma river, even from the passing train. The 

 shale in contact with the igneous rock is in general much hardened 

 and assumes a somewhat flinty aspect. 



The geology of this district becomes clear from the fact that the 

 large mass of rhyolite takes the form of laccolite, like that of the 

 Henry Mountains, described by Gilbert.* It was once covered by the 

 sedimentaries, since eroded away, and leaving the hard internal core 

 exposed at the surface. The laccolitic rhyolite crops out in the Soehi 

 valley where it forms precipitous cliffs in some places, exhibits a 

 beautiful wavy-jointed structure, w^ell seen on the western precipice of 

 the valley, and usually contains much common epidote. But the 

 locality where the typical piedmontite-rhyolity appears, is one limited 

 to a small area at Karuizawa-shinden (not the village of the same name 

 on the Usui-pass), at the upper end of the Soehi valley, about 10 

 kilometres north from Ueda, and not far from the Sakaki station. 



*G. K. Gilbert, Report on the Geology of the Henry Mountains, 1877, quoted in Xeumayr, 

 Erdgeschichte, I, p. 177. 



