TERTIARY. 



313 



field, consisting of sandy silts, white, yellow or grey in 

 colour, and the dips are usually at low angles, except 

 along the northern boundary, where they amount to 

 from 30 to 50 degrees, and point to some local dis- 

 turbance at the edge of the basin. Numerous seams of 

 coal occur, but most of them are thin and of no econo- 

 mic value. 



The chief interest of this old lake basin lies in the fact that 

 its shores witnessed the first manifestation of 



Volcanic outburst. . . . . . . . 



volcanic activity that can be shown to have 



taken place anywhere in the Shan States, since the Cambrian epoch 

 at least ; for throughout the whole sequence of fossiliferous rock 

 that has been described in the foregoing pages no trace of such 

 activity has hitherto been detected. Till now the orogenic movements 

 which caused breaks in the secmence, even those which occurred during 

 the Triassic period, appear to have been so superficial or local in 

 character that they did not affect the deep-seated sources of igneous 

 action ; but the later movements that resulted in the extraordinary 

 series of faults by which the whole of the plateau is intersected seem, in 

 this one spot, to have opened a way to the dormant energies 

 lying below. In the Irrawaddy valley and in south-western Yun- 

 nan the exhibition of these forces resulted in the outpouring of the 

 lava and ash beds that built up the great volcanoes of Popa and 

 Hawshuenshan respectively, but in the Northern Shan States the 

 display of volcanic activity wa; of a verv feeble character. 



While engaged on the survey of the Man-sang field in 1905, 

 „ , . , , Mr. Simpson noticed the occurrence, along 



Basaltic dykes. . f . , , , , , • 



its northern edge, of a number ot basaltic 

 or doleritic dykes, some of which possessed a distinctly amyg- 

 daloid structure (op. c /., p. 145). I had also met with the same rocks 

 during a rapid traverse made across the north-west edge of the 

 field earlier in the same year, but had b en unable to determine 

 whether the rock was intrusive or not in the silts, or to find any 

 centre of eruption. Later on, however, while surveying the coun- 

 try about Mong-Yai, I was struck with the 

 Focus of eruption. ° .. . 



appearance oi a conical lull named Loi I lan 



Hun, rising to about 700 feet above the plain, some (i miles to 

 the north-west of the capital, and on examining it found that, 

 while the lower part was composed of the Tertiary silts, the upper 

 part was a dome-shaped mass of columnar basalt, with dykes 



