C— GEOLOGY. 91 



concentrated. The honeycomb sandstone described by Cantrill'^ which 

 occurs at the base of I>^ along part of the North Crop of the Souili Wales 

 coalfield seems to be similar. A second exceptional case of pseudo- 

 brecciation in the Avon section occurs on a small scale in the Cleistopora 

 beds, some four feet above the base of the Bryozoa bed. In this case the 

 change has led to much sei^aration of iron and some of argillaceous 

 material. The rock affected is a crinoidal limestone. 



The prevalence of pseudobreccias in the Dj beds of the Midlands and 

 the North-west Province, as well as in the South, is one of the features 

 emphasising the uniformity of conditions which apparently prevailed in 

 the Avonian seas of England and Wales at this time. 



Though pseudobreccias are not alluded to as such in the Anglesey 

 Memoir, the term not having been introduced when the field work was in 

 progress. Dr. Greenly informs me that they are extensively developed 

 both at the top and near the bottom of D.^. They are alluded to in the 

 Memoir as mottled limestone.'^ Pseudobreccias occur in the Lilleshall 

 limestone (D.,), but, as Mr. Wedd informs me, are characteristic and 

 apparently confined to the ' White Limestones ' (upper Dj and lower D.,) 

 of Denbigh and Flint. In the southern part of the area they mark a well- 

 defined and persistent horizon near the top of the ' White Limestone.' 



Typical pseudobreccias are described by Jackson*'' from the upper D, 

 of the Miller's Dale area, Derbyshire. 



Garwood®^ describes from the D, beds of the North-west Province 

 certain structures under the names of (1) spotted beds, (2) pseudobreccias, 

 saying that the two occasionally pass into one another. The spotted beds 

 are of two types ; in the first the spots consist of small spherical patches 

 of darker limestone surrounded by a lighter matrix, the two grading into 

 one another. No constant difference between spots and matrix was 

 detected beyond the concentration of the coloured impurities in the spots, 

 this being attributed to slight concretionary action. The second type of 

 spotted bed, which is far less common, differs in the fact that the original 

 rock contained a considerable amount of sandy material. Concretionary- 

 action in this case has led to the concentration of the sand grains in the 

 matrix, while the colouring matter as in the first case collected in the spots. 

 In each case the process is believed to be contemporaneous. The structures 

 described appear to be identical with those of the South-west Province. 



Pseudobreccias are known from many other localities in the D^ beds of 

 the North of England ; thus Garwood and Goodyear ®^ describe them from 

 the Great Scar limestone of the Settle area, and Edmonds*^ from Cumber- 

 land, while they occur also in the Dale Country (Wensleydale). 



In West Cumberland they are found at many levels in the D beds and 

 extend up to D^. 



Though nodular structures suggesting pseudobrecciation are present 

 in some dark limestones, the structure is only typically developed in beds 



" Ammanford Memoir (1907), p. 69 et seq. 

 '• Anglesey Memoir, vol. ii. (1919), p. 606. 

 80 Oeol. Mag., vol. lix. (1922), p. 467. 

 " Q.J.G.S., vol. Ixviii. (1912), p. 476. 

 " /6i(/., vol. Ixxx.( 1924). 

 " Geol. Mag., vol. lix. (1922), p. 80. 



