BY W. H. TWELNETREES AND W. F. PETTERD. Gl 



by the aiigite mineral : and these two elements have com- 

 bined to form a non-vitreous massive rock of essentially 

 the same mineralogical constitution as gabbro and basalt, 

 but as regards grain and structure, intermediate between 

 the two. If we could follow this rock to its deep-seated 

 roots in the earth's crust, where the pressure was greater- 

 and the process of crystallisation correspondingly slower,, 

 we should probably find it existing there as coarsely 

 crystalline gabbro. On the other hand, we must not 

 regard its present surface as in any way its original one. 

 Much of it, as well as all the overlying rock, has been 

 removed by denudation. Admitting its intrusive nature,, 

 there are two theories of its occurrence which press their 

 claims for acceptance. Seeing that its internal structure 

 agrees closely wath that of dial^asic sills, has it spread 

 laterally from fissures covering up underlying rocks, and 

 leaving an exposed surface now owing to the removal of 

 the overlying strata ? On this hypothesis, the dolerite on 

 the tiers and the mountain tops is only a capping, and 

 shafts sunk through it would pierce the stratified sediments 

 below. The level contours of the sidimentary beds 

 abutting on the sides or faces of the Tiers, and simulating 

 infraposition, have suggested this explanation, but we have 

 had no demonstration by any actual trial. The enormous 

 thickness of the dolerite is greater than that of any sills 

 known to us. 



The second hypothesis is that what we see represents the 

 massive intra-telluric part of an immense body of eruptive 

 rock, which, as a whole, never reached the surface, but 

 which everywhere thrust out lateral dykes^ parts of which 

 we can still trace in the coal measures. Either explanation 

 is surr&unded with difficulties, which extended observation 

 alone can solve. This doleritic rock is a product of the 

 gabbroid magma; but we now proceed to notice an entirely 

 different class of rocks, those which have issued from what 

 Rosenbusch calls a theralitic eruptive magma. Deep-seated 

 rocks give the key to the relationships of the volcanic 

 ones. Hence in modern jDetrology the latter are referred 

 to or compared with their plutonic rei)resentatives. 

 Theralite is a plutonic nepheline -I- lime soda felspar 

 (occasionally potash felspar) rock, the deep-seated parent 

 of nepheline and melilite basalts. 



Ne2)helinite. — This is a nepheline-augite rock. A brief 

 examination serves to show that the long black prisms 

 which form such a striking feature are not tourmaline but 

 augite. The interstices between the jprisms are occupied 



