PSEUDOBRECCIATION IN ORDOVICIAN LIMESTONES 411 
The following may be taken as representative of several sets of 
analyses. 
Light Colored Dark Colored 
SHO) su:d hat rele BeSonic eae 1.56 per cent 1.56 per cent 
Motalironwasihe;O3. 5 ci. ac ciseo cos coe 0.16 1.94 
(BEO). sa coas Shite tee SOS OH a oeRice ©.12 0.45) 
ENO i'd) Sat aad Boe ee Bee og See ahr eee 0.06 227, 
ECO are etal recedes ier ne ek eta: sleeve 8 94.02 71.03 
HUIS CO araeatie ere siavekorare coho PTE Baleares Sas Aess 23.35 
TOE cohen ntefaec no tclotd c eack ne Catena penne 100.13 100.15 
The analyses show that nearly all the iron and alumina have 
been introduced with the Mg-bearing waters. The theory out- 
lined above is consequently untenable. It may also be pointed out 
here that while in the light-colored limestone, where the ratio of 
the MgCO, to CaCO, is approximately 1:22, no recrystallization 
has taken place, in the darker stone, where the proportion is roughly 
1:3, “dolomitization” has taken place, though the proportion 
required in a true dolomite is 1:1. This point will be again referred 
to in the discussion of the origin of the “dolomitization.” 
THE ORIGIN OF THE MOTTLING 
If we exclude true brecciation as an explanation of the mottled 
effect of these limestones, there remains only one or other of two 
possibilities. Either the dolomitization has taken place practically 
simultaneously with the formation of the limestone, or subsequent 
dolomitization has ensued by water infiltration after a great thick- 
ness of the limestone has been Jaid down, and consolidation has 
taken place. Dixon, who has made a study of the dolomitization of 
the Carboniferous limestone of South Wales, remarks on the 
difference between contemporaneous and subsequent dolomitiza- 
tion. He considers that subsequent or vein dolomitization is 
characterized by (a) larger average size and greater clearness of 
the rhombohedra, (6) inclusions of hematite, (c) association of 
dolomitization with calcification, (d) preference of dolomitization 
t Quart. Jour. Geol. Soc., XVII (1911), 477; and Geology of South Wales Coalfields, 
VIII (1907), 15 (““Memoirs Geol. Survey”). 
