THE ARCH^AN FORMATION OF THE ABUKUMA PLATEAU. 275 



a pure, fresh dunite, entirely made up of olivine grains with only 

 a little serpentinous matter j that of Akasaka-Higashino belongs to 

 the same species with exactly identical minéralogie composition and 

 habitus, but differs from the former in the particular that it abounds 

 with interposed picotite. 



VIII. Dyke-Rocks. 



A. Pegmatoplujritic Dykes. 



Along the water- worn banks of rivers and road-cuttings, there 

 are found innumerable flesh-red dykes, severally interpenetrating 

 both the younger and older granites, and even the complexes of 

 ïakanuki andtGozaisho, and it is very remarkable to observe that 

 wherever a granite occurs it is usually accompanied by this dyke rock. 



No solid rock could withstand exposure to such powerful external 

 aofencies as the alternate frost of winter and heat of summer, which 

 are excessive in, and very characteristic of, the climate of some parts 

 of Japan as may be witnessed in the [costumes of the people in the 

 half-yearly exchange from a cold -temperate to a, tropic fashion. All 

 the rocks of the Abukuma plateau, especially the granites of coarse 

 texture, are, therefore, soon reduced to an incoherent mass after new 

 excavations even within the short interval of a few yejirs. This neces- 

 sitates incessant repair of roads, in order to keep them in tolerably good 

 condition. Among such crumbling debris, the red pegmatophyritic 

 dykes, in virtue of their solidity, escape the general disintegration, 

 and stand up in bold relief in various directions crossing rivers and 

 slopes of hills, and giving thereby a peculiar character to the scenery, 

 when seen from a distance. 



Guided by prudence, the people of the district use only this dyke- 

 rock for macadamizing durable roads to which it affords a firm skeleton 



