216 KNOTT AND TANAKADATE 



the first group stations with andoubted volcanic surroundings are 

 inckided. In the third we put stations which areas characteristically 

 non-volcanic. The rocks, in this case, may be schist or granite, or 

 stratified rocks of some determinable o^eolosfic ao'e, includin"; alluvium. 

 The second group includes what may be termed the doubtful stations. 

 They may be resting on non-volcanic rocks or soil, but so close to 

 rocks of truly volcanic origin as to warrant us in regarding them 

 almost as belonging to the first group. It is highly probable, indeed, 

 that a station lying near the edge of a stretch of volcanic rocks should 

 be characterised by considerable irregularities in its magnetic elements. 

 Hence the doubtful or intermediate second group may easily compare 

 in the magnitude of its mean errors with the first group. The groups 

 are as follows : — 



Group I. — Volcanic : Shiogama, Ichinoseki, Hanamaki, Morioka, 

 Gonohe, Nobechi, Aomori, Sapporo, Nemuro, Odate, Kariwano, 

 Yokote, Shimo-innai, Shinjo, Yamagata, Ebisu, Sekiyama, Takano- 

 machi, Köfu, Hakone, Otsu, Höjö, Shimoda, Wakwan, Mëho, Pusan, 

 Nakatsu, Hichiyamiira, Nagasaki, Nanao. 



Group II. — Intermediate : Yabuki, Matsukawa, Shiraishi, Hachi- 

 nohe, Hakodate, Hirosaki, Noshiro, Akita, Kashiwazaki, Ueda, Hara, 

 Hagi, Kanömura, Koyamamura, Shioya. 



Group III. — Non-volcanic : Ishibashi, Ishinomaki, Miyako, Kuji, 

 Kiitup, Sakata, Yonezawn, Oguni, Nakajô, Niigata, Katsuura, Töga- 

 ne, Chöshi, Kioroshi, Shimmachi, Omiya, Tokyo, Shimizu, Nagoya, 

 Kamiyashiro, Nagnhama, Hyögo, Tokushima, Köchi, Minabe, Okaya- 

 ma, Hiroshima, Fukuoka, Sagancjseki, Miyazaki, Yatsushiro, Hamada, 

 Matsue, Imaichi, Maizuru, Obama. 



The mean proljable errors for the ditferent groups are given in the 

 following tables — the first being based on the results for all the sta- 

 tions, the second taking account of the fifty selected stations only. 



