I 



I 



BY H. I. JENSEN. 853 



A very interesting dyke occurs on the western side of Mt. 

 Conowrin, exposed by a landslip a few years ago. 



The most conspicuous dyke in the district is, however, the one 

 which has given rise to the anticline in the sandstones of a rail- 

 way cutting half a mile north from Beerburrum Railway Station. 

 This dyke has forced its way along a bed of shale interbedded 

 with the sandstone, the lava having carried some of the shale 

 before it in its path. The lava has seemingly come from the 

 S.E., so that we get a mass of altered black shales, about 40 feet 

 in thickness, exposed in section on the western flank of the cutting, 

 whilst the original shale bed showing on the eastern flank has 

 only a thickness of two or three feet at the most. On this side 

 the trachyte dyke does not show. Evidently the lava has come 

 diagonally upwards. (Plate xlix., fig. 5). 



The finely crystalline nature of the dyke rocks, as well as their 

 close resemblance structurally and mineralogically to the trachytes 

 of adjacent peaks, seems to me to show that they are derived 

 from the same source, and contemporaneous. From their texture 

 it is evident that they consolidated near the surface, and hence 

 it appears that the amount of denudation undergone by the 

 Triassic rocks since the trachyte eruptions has been small. 



(4). Possible Laccolites. — From Medway's Mountain on 58v 

 Canning westward, an area including selections 2v, 58v and 86v 

 has trachyte rock underlying the surface soil. The trachyte here 

 is considerably more coarse-grained and more ferruginous than 

 that which has found vent in Medway's Mt. On weathering, it 

 turns brick-red. Along the right side of the Durundur road, 

 which crosses selections 2v and 86v, a trachyte outcrop many 

 chains in width and nearly a mile in length may be traced. It 

 does not reach an elevation of more than 10 to 12 feet above the 

 surrounding country. The rock weathers into huge boulders in 

 much the same way as granite, and in mineral composition it is 

 analogous to the trachyte of Mt. Beerburrum. Within a radius 

 of half a mile from it the soil is very poor, ashlike and caking, 

 typical of decomposing trachyte. This is probably a laccolitic 

 mass which has consolidated under a bed of sandstone or loosely 



