These assay results are in line with the observations of others (3b•0•12) on the instability of B12b in the presence of certain reducing agents and assay media constituents such as thiamin and ascorbic acid. The apparent differences in microbiological assay attributed to vitamins Bi2a and B12b7 may well be the result of assay methods.Acknowledgments.-The production and isolation of vitamin Bi2b is obviously the result of group effort by many individuals. We are grateful to Dr. G. F. Cartland for active direction of the program, to Dr.
The effects of the Variscan orogeny on the Upper Palaeozoic succession are recognizable throughout central and southern Ireland. The deformation can be zoned using structural styles and trends which illustrate a northward decrease in deformation intensity coupled with increasing control by pre-Variscan structures in the basement. The most intense deformation occurs to the south of the Dingle-Dungarvan line, where shortening approaches 50% as a result of layer-parallel shortening, followed by buckling and finally thrusting. The deformatipn is considered to have developed above a shallow-dipping sole thrust which has been located offshore by the SWAT seismic reflection profiles. Cross-faults, which are common throughout the orogen, resulted from N-S shear couples; this is consistent with their origin as accommodation structures due to differential shortening along strike. The original shape of the Munster Basin has exerted a control on the distribution and overall facing of the major structures. The sole thrust extends to the north of the Dingle-Dungarvan line and the structural zonation of the orogen is due to variations in the stratigraphic level of this thrust. The Leinster Massif is considered to be essentially autochthonous; slip on the sole thrust decays towards the Massif which acted as a rigid, passive block and was partially responsible for the arcuate trend of the orogen.
Previous work in the Irish Variscan orogen has emphasized the significance of the major folds in the overall structure. A series of three balanced sections through the orogen reveals that thrusts are an equally important element of the orogenic structure. The 'thrust front' is not a fundamental structural line delimiting different deformational zones as previously proposed. Major thrusts occur along the 'thrust front' between Killarney and Mallow, but are absent east of Mallow. This change in character of the 'thrust front' is a function of a change in the stratigraphic level of the sole thrust to the orogen and the current erosion level. The deeper level of the sole thrust produces the numerous Silurian cored inliers to the NE of Mallow. The balanced sections show that the overall bulk shortening in the orogen increases from 33% in the east, to 42% in the central areas. This increase may be related to the arcuate nature of the orogen.
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