LACCOLITES AND BYSMALITES. 187 



to force water up an ordinary jointed pipe from sea-level to the 

 top of Mount Blanc, the pressure of some 16,000 feet being 

 about the same as that of the cover of rock, of specific gravity 

 2.7, which the molten rock was supposed to have driven upwards. 



In South Africa the case is quite clear. The Karroo dolerites 

 have not lifted their cover, but have melted and absorbed the 

 rocks along planes of easy solution. At the same time, the 

 evidence in South Africa must not be taken to disprove the unlift 

 theory in other countries where the conditions are different. As 

 a rule dykes, sills and laccolites are frequent in shale formations 

 and are very sparingly found in sandstones. This, to a certain 

 extent, if not exclusively, is due to the fact that rocks with 

 several diverse constituents are more readily melted, or dissolved, 

 than ones with only one predominant mineral, as in sandstones, 

 the other constituents forming fluxes, which lower the melting 

 point. In South African rocks, dykes in Table Mountain sand- 

 stone, Kheis quartzites and Black Reef quartzites are exceedingly 

 rare, whereas in theshaly Pretoria series and Karroo beds they 

 are very common. In limestones they are moderately frequent, 

 but of small dimensions. Under favourable conditions, where 

 dolerite, for instance, invades a sandstone formation, then the 

 uplift would tend to develop. For some such reason the lacco- 

 lites on the western edge of the North American prairies have 

 developed, and strings of them have probably been responsible 

 for the mountains of " block uplift," the Wahsatch, Sevier and 

 other plateaux of Utah and Colorado. Even in the American 

 laccolites, types occur which have partially dissolved out their 

 cavities, such as in Cross' compound laccolite of El Late Mountains 

 in Colorado. In others, as in the Judith Mountains in Montana 

 and Mount Hillers in the Henrv Mountains, subsidiary sheets 

 have been sprung in between the sediments which have been 

 uplifted like in the lit-par-lit injection round granite massives; 

 in these cases the sheets are of considerable dimensions and often 

 break across the strata, enclosing slabs of rock, which then become 

 Xenoliths, and which may become absorbed in the magma. In 

 this way a process of stoping down the roof may considerably 

 enlarge the cavity. No such perfect fusion of the sedimentary 

 rocks which once occupied the space where the laccolites now 

 lie is found in America as in South Africa. Nevertheless, the 

 gradation from the one form to the other shows that there is 

 no essential difference in their nature; hence it is perfectly justi- 

 fiable to use the term for our enlarged sills occupying cavities 

 which they have melted out for themselves as it is for the 

 American type, which have forced the cavity open by hyydrostatic 

 pressure. 



Evidence that dykes absorb the rocks into which they are 

 thrust is everywhere available in the Karroo. On the bare slopes 

 of the Nieuweveld in Beaufort West the great dykes and sills of 

 dolerite are seen traversing the sedimentary rocks with no dis- 

 turbance whatever of the planes of bedding. Fissures do occur 

 as in the one filled in with coal at Leeuw Rivers Poort, west of 



