In Hokkaido, there are many ultra-mafic igneous bodies along the two parallel tectonic zones which are named the Kamuikotan zone and the Hidaka zone respectively., These tectonic zones compose the geological back-bone of Hokkaido, and are regarded as the product of a Cretaceous orogenic movement which is called "Hidaka Orogeny"., It is believed that the Hidaka zone may be an orogenic deep core and the Kamuikotan zone its westward over-thrusting zone., Many common characters have been noticed in the ultra-mafic igneous rocks of both tectonic zones., But contrasting features have also been observed in them., One of the most contrasted features is that the ultra-mafic rocks of the Kamuikotan zone are almost wholly serpentinized, on the other hand, those rocks are always in the unaltered peridotitic state in the Hidaka zone., In the Horoman region, at the southern end of the Hidaka tectonic zone, occurs the largest peridotite intrusive mass of the zone., The rock-forming minerals of this peridotite have an orientated arrangement, so the flow structure or layered structure are well marked in all parts of the mass., Judging from the arrangement of these flow structures, it seems that a large dome structure has existed at the centre of this peridotite mass, as presented in Fig 1., There are various kinds of rock facies in this peridotite mass, such as dunitic, pyroxene-peridotitic, and plagioclase-peridotitic facies etc., and each rock facies is developed in accordance with the flow structure., So, in a cliff, obvious appearance caused by the alternation of different kinds of rock facies seem clearly to be stratifications., (Plate 1)., -And their mineralassemblodges in each facies follow., 1., Olivine -(dark brown spinel) 2., Olivine -rhombic pyroxene-monoclinic pyroxene-(light brown spinel) 3., Olivine -rhombic pyroxene-monociinic pyroxene -(green spinel) 4., Olivine -monoclinic pyroxene-plagioclase-(black spinel) Optical characters of each mineralare given in Table 1., In this paper, the writer described the petrographical characters of each rock facies as a basis for further development of the knowledge about the Horoman peridotite mass., Further studies, especially concerning the intrusive tectonics, will be published in the near future by collaborators of the present writer.,
