Results of X-ray textural studies of dolomite and the clay minerals in the Marl Slate and of siderite from the top of the Mansfield Marine Band are interpreted in terms of the possible diagenetic development of the re-crystallized carbonates in these two rocks. From computer processed (1014) fabrics of the two trigonal carbonates it is deduced that there is a c-axis preferred orientation concordant with the normal to the stratification. An early diagenesis in this oriented mode must be a function of both a threshold stress difference in a non-hydrostatic stress field (in effect a necessary minimum depth of burial) and also of the presence of connate solutions. The existance of a minor percentage of dolomite oriented with the rhomb faces parallel to the stratification probably reflects an even earlier crystallization within the top few metres of sediment, when the confining pressures and pressure differences were of insignificant amplitude. It is concluded that a higher rate of sedimentation associated with the Coal Measure sequence is responsible for the singular preferred orientation of the siderite.