72 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Dec. 3, 



The strata forming the cliff were found, where they were cut 

 through by a shaft sunk, by the direction of the author, for the pur- 

 pose of examining their relations and position, to lie in the following 



order : — 



Ft. in. 



1. A small quantity of alluvium between the cliff and the 



steep part of the hill. 



2. Gravel composed of the detritus of trap-rocks, containing 



soft clay and laminae of indurated shale 2 6 



3. Gravel of indurated shale in an argillaceous matrix .... 1 6 



4. White porcelain-clay 2 



5. Clay containing pieces of the indurated fossiliferous shale 1 6 



About 300 yards to the north of the excavation, where the course 

 of the stream has a direction from N.W. to S.E., the cliff is higher. 

 At its base occurs a dome of basalt, which has bent the beds 

 upwards into the form of a parabolic arch. Immediately above lies 

 a gravel formed of the rounded detritus of trap-rocks, resembling that 

 found in the shaft, 6 feet in thickness ; and above this gravel, beds of 

 indurated shale, in thin leaves, alternate with clay, forming a bed 

 1 foot 6 inches thick. Above this lies a gravel composed of disin- 

 tegrated trap -rocks. 



The laminae of shale in the thin beds of clay just mentioned re- 

 semble in substance and colour the fossiliferous strata below, but 

 they are too disintegrated to contain perceptible traces of fossils. 

 They are very distinctly seen in the cliff for a considerable distance 

 to the north, but eventually disappear beneath the bed of the stream, 

 about one verst to the north of the excavation made to examine 

 the position of the strata. 



The surface of the plain where the igneous rocks are seen seems 

 to be composed of a detritus formed by their disintegration. It is 

 bounded eastward by a high ridge of hills rising gradually from the 

 plain, and extending northward and southward. This ridge is com- 

 posed of fine-grained granite, and is covered in places with fragments 

 of lava and scoriae. Remnants of metamorphic rocks occur to the 

 east of this ridge amongst fragments of basalt, and further eastward 

 ridges of clay- slate crop out with a westerly escarpment. 



At the foot of the ridge of fine-grained granite, and about 20 

 versts south of its southernmost extremity, lies Odon Tchalon, the 

 " Variegated Uountain." The summit of this mountain is formed of 

 three cone-shaped hills, composed of a very crystalline garnet- 

 bearing granite, containing abundant nests of yellow topaz and 

 aquamarine. Beryl and amethyst are also found in it, some of the 

 latter being very valuable. 



The inference it seems most natural to draw, on consideration of 

 the general position of the strata which appear on the surface of the 

 surrounding country, and from the dip of their beds, is, that the 

 garnet-bearing granite of Odon Tchalon represents the nucleus or 

 centre of this eruption ; and that the ridge of fine-grained granite, 

 commencing immediately to the north of it, and running nearly north 

 and south, is the line of the eruptive force that formed the Ncrtchinsk 



