38 GEOLOGICAL RESEARCHES IN 



Beds of another rock occur, of brick-red and brown colors, and having an earthy- 

 base, with small, brilliant crystals of glassy feldspar and grains of pellucid quartz, 

 and inclosing small fragments of other rocks. 



This deposit is visible on the southern flank of the spur between Kalgan and 

 Siuenhwa, underlying the terrace loam in horizontal beds (Fig. T). 



At the base of the high hill north of Kalgan the tufa beds are seen to dip under 

 tlie porphyry at an angle of about 45° (Fig. 8), and trending west they form a series 

 of detached hills. On the roads leading to Tutinza, Teutai, and Siwan, they are 

 traversed by a perfect network of dykes of the porphyry, which rock also caps the 

 summits of the hills, its vertical clifi's and outstanding dykes giving them a bold and 

 castellated appearance. 



Although no analyses of these rocks have been made, there is, I think, little 

 doubt that we have here to do with a trachytic porphyry and its tufas. 



Volcanic Formation of the Plateati. — The southern elevated edge of the Great 

 Plateau is formed, between the 112th and 115th meridians, of an immense lava bed. 

 How much further it extends beyond the limits given above, or how large its 

 breadth may be toward the north, is unknown ; I have only tried to indicate on 

 the map the region which I observed it to occupy. Its breadth is, in places, not 

 less than forty miles, and this may be only a fraction of the real width. 



The thickness of the formation is, necessarily, very variable as it fills the in- 

 equalities of what was once a mountainous country. At Hanoor it seems to be not 

 less than fifteen hundred feet thick, and the same may be said of it in other locali- 

 ties visited, while we have seen it in places represented by only a thin sheet, 

 covering the metamorphic schists, where these rise to near the surface. 



The rocks of this formation may be classed under two types — the one basaltic, 

 the other trachytic. 



The basaltic rocks were observed more particularly near Hanoor and to the N. 

 E. of that place. Both compact and finely crystalline varieties occur. They are 

 generally, especially the latter variety, poor in olivine and contain here and there 

 crystals of basaltic hornblende. 



At many places in the neighborhood of Hanoor, fragments of a cellular variety 

 occur on the sides of the valleys, in a manner that Avould seem to indicate, that 

 there is a horizontal bed of it, marking the plane of contact between two flows of 

 lava. 



The rocks of the other type are throughout crystalline, though often the texture 

 is very fine, and are generally porous. In color they vary from black to dark gray, 

 while some varieties, especially when weathered, are light gray. In some instances 

 hornblende, or augite, enter abundantly into the composition of the rock, but more 

 generally it seems to consist almost exclusively of white or yellow, triclinic feld- 

 spar with greasy lustre, partly in tabular crystals, partly massive. Scattered through 

 this mass are minute specks or grains of a dark to light green mineral, with glassy 

 lustre and conchoidal fracture, harder than the knife when fresh, soft and resinous 

 in lustre when altered. The feldspar is probably oligoklas. A characteristic 

 feature of the different varieties of this rock is the extreme rarity or total absence 

 of maOTietic iron. 



