DEVONIAN SYSTEM : PLATEAU LIMESTONE. 



193 



(2) The mineralogical changes that the rocks have undergone 

 are similar to those which have affected the limestones of 

 modern coral reefs, viz., the dolomitisation of the rock 

 and the disappearance of the organisms that originally 

 composed them. 1 



(3) The practically complete dolomitisation of the great mass 

 of the Plateau Limestone is an indication that deposi- 

 tion took place in a slowly subsiding area, whereby 

 sufficient time was allowed for the mineralising agents 

 present in the sea water to have complete effect. 



(4) Conversely, the fact that the Permo-Carboniferous band at 

 the top of the formation has not been altered into dolo- 

 mite is an indication, either of more rapid subsidence, 

 or what is more likely, seeing that it is followed by a 

 break in deposition, of a comparatively rapid upheaval 

 above the surface of the sea. 



(5) Some of the dolomite in the rocks is of a secondary char- 

 acter, and has been deposited directly from solution in 

 fissures and cavities. 



(6) The specific gravity of the limestone may perhaps be used 

 as a rough guide in estimating the amount of dolomitisa- 

 tion that has taken place. 



A very constant feature of the Plateau Limestone, and one 

 which requires some description and explana- 

 ofd^o^tea str,,cture tion, is the extraordinary manner in which it 

 has been crushed, to such an extent indeed 

 that it is difficult to find a piece sufficiently large for a hand 

 specimen that is not traversed in all directions by a network of 

 veins or fissures filled with secondary calcite or dolomite ; and 

 when the filling of the veins has been leached out, the rock is in 

 so pulverised a condition that a single blow with the hammer 

 is often sufficient to reduce it to a heap of ballast or road-metal. 

 These veins are frequently 1 faulted ' across by others, and ' slicken- 

 sided ' surfaces are not uncommon, indicating that differential 

 movements have taken place in the mass, and that the movements 

 have been spread over a period sufficiently long to allow one set 

 of veins to be filled with solid material before the rock was again 

 broken up and a fresh set formed. I do not refer here to the 



1 (!. <!. Cullis, The Atoll of Funafuti ; Report, Coral llei-f Committor) Ho>i, Sue. London 

 1904, Sect. XIV, p. 392. 



