The rocks from the Chokai volcano are classified into two series, the high-alumina basalt series and the calc-alkali rock series. The calc-alkali rock series is also divided into two types, the hornblendebearing type and the hornblende-free type. The high-alumina basalt series has intermediate character in chemical composition between the alkali rock series and the tholeiite series, especially as concerned to the alkali contents. It is worthy of note that the high-alumina, basalt series of this volcano is richer in alkali contents, especially in K
2O
_??_, than those from other districts.
The calc-alkali rock series is derived from the high-alumina basalt series by assimilation and shows also intermediate chemical character between the calc-alkali rock series derived from alkali olivine basalt magma and that derived from tholeiite magma. Difference of the chemical composition is found between the hornblende-bearing type and hornblende-free type; i. e., the former has higher MgO/FeO+Fe
2O
3 ratios than the latter. Similar relations is also found in many other districts in Japan. Probably the appearence of hornblende depends upon not only contamination, but also the difference of chemical composition of the original magmas before contamination.
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