m DESCRIPTION OF THE STRATA. 



in the Speeton shale, are of a chalky nature. The septaria are not 

 imfrequently in a shape resembling the tortoise, being oblong, with 

 the lowest side flat and the upper rounded; and when they occur with 

 the septa or divisions, from whence they have their name, appearing 

 through their outer crust in distinct lines or markings, the resem- 

 blance is so great, that they are actually collected and sold as petri- 

 fied tortoises. 



This shale, like most other beds of slate-clay, also abounds with 

 crystals of seleuite. The most beautiful specimens are found at the 

 oldest clay pit at Knapton. They sometimes occur in flat rhomboidal 

 crystals, with bevelled edges: but more frequently in oblong flat 

 pieces, but with the angles and facets, at their terminations, corres- 

 ponding with those of the rhomboidal pieces. In the same clay pit, 

 the authors found some good specimens of those branching tubes, not 

 unlike the roots of furze, formed by oxide of iron, percolating through 

 the clay in a fluid or moist state. It is proper also to observe, 

 4hat iron pyrites abounds in the nodules that occur in tMs shale, as 

 in almost all the shale beds of the district. 



Had this bed occurred above the chalk, instead of lying under it, 

 there is no doubt that it would have obtained the name of the London 

 clay; for, as far as can be judged by the descriptions given of the 

 latter, the two beds are remarkably similar, b»th in their substance 

 and their contents. It is not, however, in this district alone, that 

 shale of this description is found under the chalk. In Cambridgeshire 

 the grey chalk rests on an extensive bed of blue clay, provincially 

 called gault. The grey chalk is there of much greater thickness 

 than in our Wolds, as it composes the greatest part of the hills of 

 Cambridgeshire. It is also of a much harder quality; yet, like our 

 grey chalk, it becomes argillaceous, soft, and of a greenish hue, 

 where it joins the. clay. It contains pyrites, but appears to differ 

 I'rom our lower, chalk, in being entirely grey, without any mixture of 



