LACCOLITES AND BYSMALITES. 



By Prof. Ernest H. L. Schwarz, A.R.C.S., F.G.S. 



The term laccolite was first used by W. K. Gilbert in 1879, 

 in his Report on the Geology of the Henry Mountains, Utah. A 

 laccolite is a mass of intruded rock which has come into its 

 position in a molten state through supply dykes which communi- 

 cate with the lower portions of the earth's crust. When the idea 

 was first promulgated dykes were supposed to have wedged them- 

 themselves along fissures which opened for the admission of the 

 molten rock and, then, if a suitable horizontal plane of parting 

 presented itself, the molten rock would follow along this line of 

 weakness and produce a " sill." New rock thrust in between 

 bedding planes or other horizontal planes in sedimentary rocks 

 necessitates the uplifting of the overlying strata. A laccolite is 

 a local bulging of a sill, and the uplifting was originally supposed 

 to have been concentrated in one particular spot and a local 

 uplift to have occurred. It is very plain that the original Henrv 

 Mountain laccolites have domed up the superincumbent strata, 

 though these amounted in thickness from 7,000 to it, 000 feet, 

 according to Gilbert. The mode of intrusion was that a vertical 

 fissure opened and molten rock came up it, driven by hydrostatic 

 pressure from below ; at a certain horizon the more or less vertical 

 fissure stopped, and the magma continuing to be pumped up, 

 drove upwards the beds above where the vertical fissure came to 

 an end, and such was the force impelling the molten rock that 

 two miles of cover were readily bent upwards. In the case of 

 a sill such as we have examples of in the Karroo, a small supply 

 dyke communicates with the main mass, which is often hundreds 

 of square miles in area and three to four hundred feet thick. 

 The lowest sills in the Beaufort West district are under a cover 

 of not more than 2,000 feet of sediment ; but the sills are arranged 

 one above the other, and the topmost sill must have been under 

 considerable cover, otherwise the molten rock would have burst 

 through to the surface and have formed volcanoes. A conserva- 

 tive estimate, therefore, of the cover lifted by the lowermost sills 

 in Beaufort West, supposing the uplift theory is correct, — which 

 ] doubt, — would be 5,000 feet. Some of the sills are more than 

 2,000 square miles in area, and over this whole area the over- 

 lying sediments are supposed, on the uplift theory, to have been 

 raised three or four hundred feet. There may have been manv 

 supply dykes instead of one, but even in that case the mechanical 

 difficulties of transmitting such a huge uplifting force from the 

 supply dykes to the further corners of the sills, would seem to 

 disprove the theory. The crust of the earth is supposed to be 

 Assuring to allow the entrance of the molten magma, and at the 

 same time it is supposed to allow the transference of a gigantic 

 force by hydrostatic pressure. It would be the same as trying 



