2i8 The American Geologist. October, i904. 
consideration in detail and fasten my attention upon the separation by 
the native imperial geologists into north Japan and south Japan, by 
which the boundary is placed in the fault cleft which limits the fossa 
magna on the west side. The results I have sought to enter upon the 
accompanying map. (See Plate XIV.) 
A. FUND.\MENTAL STRUCTURE OF NORTH JAPAN. 
On the east side of North Japan occur two elliptical mountam 
masses similar to each other in form but dififerent from each other 
in many respects, which following Naumann's example are designated 
by the Japanesel geologists as the Abukuma mountain region and the 
Kitakami mountain region. 
The Abukuma mountain region forms according to Koto, who 
has investigated the difficult structure with care, an acute rhomboidal 
horst-like mountain block extending from north to south 150 kilometers 
in length, and 45 kilometers in average breadth. The undulating upper 
surface has a mean elevation of 400 meters, with an extreme elevation 
of 933 meters, and ascends gently toward the east from the longitudin- 
al valley bounding it upon the west ; and on the east side, 500 to 600 
meters high, it falls away quite steeply upon a coastal terrace two to 
four kilometers broad covered by the uniform Hara steppe, which in 
the basement consists of Tertiary sediments and is covered with 
ancient coastal debris of granite blocks, sand, and gravel. The sea 
is now at lower levels making an attack and eroding this terrace ; 
hence the narrow strand is accompanied by low but steep clififs. 
Through depressions on the west and east side the mountain body 
proper raises its gently undulating upper surface as a horst. Deeply 
incised cross valleys have brought about a transverse division of it. 
The entire mountain range consists according to Koto of Archaean 
rocks which fall into three divisions. The lower, designated Lauren- 
tian, consists of plutonic rocks which he has regarded as having become 
schistose partly through plastic and partly through rigid (firm) defor- 
mation. In the second, or Takanuki series, which consists essentially 
of gneissose mica schists and titanite-bearing amphibole schists, two 
stages are distmguished of 5,000 to 5,500 meters thickness, while the 
uppermost division. Gozaischo series, shows essentially ampbibolytes and 
mica schists estimated at about 10,000 meters thickness. All strike 
directions are near the meridian, but always with a deviation toward 
the NW, hence in the mean about N by W. The same holds true for 
the nmnerous dike phenomena of ancient eruptive rocks and for the 
direction of the rectilinear meridional side of the rhomboid. Even if 
the numbers are considerably exaggerated it is yet clear that the Arch- 
aean formations are very heavy. It is, however, to be observed that the 
Japanese imperial geologists are inclined to identify the Gozaisho stage 
with their Sambagawa stage, which, to judge from its rocks, corre- 
sponds to the Wutai beds in China, that is to say, the Algonkian stage. 
As a closed mass the Abukum:i mountain region reaches its north 
limit a short distance north of the 38th parallel of latitude. Approxi- 
