THE ARCHJGAX FORMATION OF THElABUKUMA PLATEAU. 273 



intruding basic magma in the formation of olivine in the latter. In 

 several of the papers by Professors Judd and Bonney it is clearly 

 pointed out, how fluctuating are the relative quantities of the com- 

 ponent minerals of basic rocks. It is frequently observed that, by a 

 slight textural modification, dolerite (diabase) gradually passes into 

 gabbro, by the disappearance of feldspar, into picrolite, by the dis- 

 appearance of augite, into troctolite, and, by the disappearance of 

 olivine, into eucrite ; while by the excessive development of accessory 

 constituents like enstatite and picotite fresh varieties such as Iherzolite 

 may arise. 



The prevalent rock, and at the same time the foundation stone of 

 the Abukuma, is of intermediate chemical composition, to, i. e. the 

 syenitic granite, which is specially liable to form transitions to tonalité, 

 diorite, monzonite, and eventually to gahhro ; and it seems to me highly 

 probable that the olivine rock in question represents the extreme basic 

 member of the syenitic magma, which has sent out various offshoots in 

 the form of apophyses from the main mass in the bosom of the earth. 

 Being influenced by cooling surfaces which must have been com- 

 paratively large and many in such narrow fissures, a selective crystal- 

 lization has been induced in the magma, whereby olivine has been 

 formed at the expense of the other minerals ; and thus we have 

 several dykes filled u]) with rocks of a peridotic composition. 



The olivine rock which came under my notice in the field should 

 be grouped together with amphihole-picrite. It is always very massive 

 in appearance, rarely exhibiting banded structure. Its irregular, 

 rounded blocks are often covered with a thin coating of a deep-red 

 colour due to decomposition, beneath which, however, the rock is 

 surprisingly fresh and of a dark greyish-black. Under the microscope 

 it seems to consist of a sheaf-like, colourless hornblende (tremolite) 

 piercing through the granular aggregates of olivine. The latter is 



