584 Y. OINOUYE 



May, 1912, and in May, 1913. Many interesting facts were ob- 

 served which will be published later in another paper. Here 

 attention is to be called only to certain peculiar cones formed on 

 one of the mud flows. 



The main eruption of the volcano caused the formation of 

 forty-five small explosion craters on its northern slope. These 

 craters extend from east to west in two zones along Lake Toya, 1 

 north of the volcano. During the first few months after their 

 formation innumerable bombs and considerable quantities of 

 sand and ashes were blown from every craterlet. From five of 

 them mud flowed at different times, the flow from a small crater 

 at the southern foot of the parasite cone Xishi-Maruyam being 

 especially interesting. This crater is located on a gentle slope of 

 about five degrees, and is 100 meters in diameter. For twenty 

 days it intermittently threw out columns of hot water, occasionally 

 mingled with mud, to a height of about 60 meters. Approximately 

 two hundred eruptions occurred per day at intervals of from three 

 to thirty minutes. A mass of mud, estimated by the writer at 

 230,000 cubic meters, spread out in a sheet averaging 1 . 5 meters 

 in thickness, over an area 200 by 700 meters. It covered a farm, 

 where it destroyed a thousand apple trees and other crops, and 

 pushed three houses in the direction of the lake and finally 

 destroyed them. 



The mud consists mainly of plagioclase, hypersthene, augite, 

 magnetite, and hematite, and resembles the material of the sea 

 sand at the west foot of Mount Usu. It differs, however, in also 

 containing fragments, from the size of peas to that of nuts, of com- 

 pact gray to coarse black andesite. These fragments are not usually 

 exposed at the surface of the mud, having sunk on account of their 

 greater size. 



The materials thrown out by the crater were highly heated and 

 sticky at the time of their eruption, and contained a great amount 

 of water and gas. For several months the flow continued steaming, 

 but as time passed and the moisture and gases became exhausted, 

 it ceased, and the mass became harder and harder. A year after 



1 Lake Toya is a depression lake, according to T. Kato {Report Earthquake Investi- 

 gation Committee, Vol. LXII). 



