June, 1907.] THE VICTORIAN NATURALIST. 27 



porphyritic felsites, banded felstones, massive felstones, «&c. 

 These rocks were all classed as acid lavas by the two geologists 

 just named. The term rhyolite does not appear to have yet been 

 applied to them, but both from field and microscopic examination 

 the characters implied by this term are most admirably fulfilled by 

 the greater part of the series. It is hoped later to give the result 

 of a more detailed petrological examination of these rocks. In 

 further referring to these rocks the term rhyolite will be used, as 

 it appears more definite and accurate than the older and often 

 loosely applied terms. These rhyolites can be traced from the 

 extreme southern limit of the Upper Paljeozoic series, a few miles 

 to the east of Glenmaggie, continuously along the strike in a 

 northerly direction for over 40 miles in a straight line past the 

 Snowy Bluff. Very probably a much greater northern extension 

 will be noted when the country between Mt. Howitt and the Snowy 

 Bluff is examined. The greatest observed thickness of these vol- 

 canic rocks is on the western face of Mt. Wellington, where up- 

 wards of 2,000 feet is exposed. The first appearance of this pile 

 of igneous rock from the western side, with, in part, its massive and 

 somewhat course-grained porphyritic rocks, suggested that a vol- 

 canic stump might be represented here. Such a view might be 

 read from the diagrammatic section given in the paper dealing with 

 Lake Karng (10). Wider examination, however, of the surround- 

 ing country shows that much of the porphyry represents rhyolitic 

 lavas, forming one series with the overlying sediments. On the 

 top of Mt. Wellington, near the western edge of tlie main southern 

 mass, a small occurrence of rock somewhat resembling an agglom- 

 erate was noted. The question, however, must be left as an open 

 one, but it seems very probable that both features may be 

 represented by the main southern mass of the mountain. 



A diagrammatic sketch, approximately east and west across the 

 country, and some miles to the north of the main Wellington mass, 

 probably indicates the broad structural features of the area. 



On the soutiiern shore of the lake, towards the east end, and a 

 few feet above the water line, a very small exposure of sedi- 

 mentary rock was visible beneath the porphyry, and showing little 

 or no signs of contact alteration. It appears likely, therefore, 

 that the overlying lavas are here resting on a Lower Palaeozoic 

 platform at about the level of the lake. Towards the top of the 

 mountain the dip of several successive lava sheets to the east can 

 be clearly seen, and their colunmar jointing forms a noticeable 

 feature on the north-western cliffs of the mountain. On the table- 

 land of the main southern portion the rhyolites show a striking 

 flow structure. The prevailing colour of the rock is a light grey 

 on the weathered surface, but m places extremely fresh faces are 

 exposed, showing a dark to black almost glassy texture, with 

 conspicuous white to grey lines of flow. 



