514 Notices of Memoirs — E. A. N. Arher- — The Ardwick Flora. 



or serpentine after forsterite, tremolite, diopside, and brucite. The 

 dominant carbonate of the marble is calcite, but dolomite occurs in 

 variable quantity. The amount of dolomite decreases as the total 

 amount of the magnesian silicates and brucite increases. The 

 original dolomite contains a variable amount of silica in the form 

 of chert. 



When the altered rocks are examined under the microscope it is 

 seen that forsterite, serpentine, and tremolite are invariably associated 

 with calcite, but that diopside is sometimes associated with dolomite. 

 These facts of paragenesis can be easily accounted for if we assume 

 that the silica of the original dolomitic rock has combined with the 

 bases of the carbonate, and preferably with the magnesia, for diopside 

 is rare. Thus forsterite, a magnesian silicate, cannot have been 

 formed in the dolomite without the liberation of lime, and con- 

 sequently we find either detached crjj^stals of forsterite surrounded b}' 

 aureoles of calcite in a matrix of dolomite, or, when forsterite is 

 abundant, a simple aggregate of forsterite and calcite. The formation 

 of tremolite in which the ratio of Ca : Mg is 1 : 3 also implies the 

 separation of lime from magnesia ; and it is invariably found, like 

 the forsterite, in direct contact with calcite. But in diopside the 

 ratio of Ca : Mg is the same as in dolomite ; so that in accordance 

 with the principles above explained we should expect to find these 

 two minerals in contact, and this has been observed. 



The above facts clearly point to the conclusion that the cherty 

 dolomites have been dedolomitised by the formation of magnesian 

 silicates. Carbonic acid has been driven off, but the ratio of the bases 

 has not been disturbed. The ratio of CaO : MgO in the altered as 

 in the unaltered rocks is approximately 1:1. 



But dedolomitisation has also been produced in another way. 

 Certain varieties of the marble are composed of calcite and brucite. 

 The brucite is probably a pseudomorph after periclase, just as the 

 serpentine is a pseudomorph after forsterite. We are therefore 

 compelled to conclude that, under the conditions which prevailed 

 during the intrusion of the plutonic rocks, the carbonic acid freed 

 itself more readily from the magnesia than from the lime; thus, in 

 the absence of silica, giving rise to the formation of periclase and 

 converting the orio;inal dolomite into an affg-reo-ate calcite and 

 jiericlase, the latter mineral subsequently being changed to brucite. 

 The resulting rock is identical with the well-known predazzite of the 

 Tyrol, which was probably formed in a similar way. 



VI. — On the Fossil Flora of the Ardwick Series of Manchester. 

 By E. A. Newell Arber, M.A., F.L.S., F.G.S.i 



rpHE Ardwick Series of Manchester forms the highest portion of 

 X the Coal-measures of the great South Lancashire Coalfield. 

 The plant - remains in the shales associated with the Spirorbis 

 Limestones of this series have been already mentioned or described 

 by Williamson, Salter, and especially by the late E. W. Binney. 



' Abstract of paper read before the British Association, Southport, Section C 

 (Geology), September, 1903. 



