396 REPORT — 1856. 



There are some cases in which the cleat varies in its direction from the 

 normal strike, and degree or inclination, even (as I have been informed*) in 

 different parts of one bed of coal. From frequent inspections of cleat in 

 its ordinary state and near trap dykes and near faults, I conceive that no 

 doubt can exist of its being a peculiar structure, more resembling the effect 

 of aggregation under polar attractions than anything else. Coal affected by 

 it is not properly 'cleavable' like slate, but actually cleft into numerous 

 parallel, nearly vertical tables, whose general direction is remarkably uniform 

 amidst many variations of other concomitant conditions. 



Neither heat nor pressure seem to be specially indicated by the phaeno- 

 mena of cleat, which on the whole most resemble the jointed structure of 

 rocks, where that is manifested on the smallest scale and in greatest regu- 

 larity, i.e. where the dips of the strata are most uniform, and all the conco- 

 mitant conditions are the most regular. Joints, like cleat, have very preva- 

 lent directions in given districts, and inclinations to the strata tending to one 

 angular value in one bed. In parallel beds of the same mineral nature and 

 in the same series of strata, their strike and dip are often the same. In beds 

 of a different mineral nature joints vary in character ; and in a given series 

 of argillaceous, calcareous and coarse arenaceous rocks, we may find many 

 plane close joints in the argillaceous beds, inclined 70° ;H to the strata ; a 

 few large continuous Jissiires in the limestone nearly perpendicular to the 

 beds; and a varying number oi irregular rents in the sandstone. 



Addendum (1857). — Very lately Professor Haughton has instituted accu- 

 rate measures and calculations founded on the distortion of fossils in cleaved 

 rocks, and has obtained numerical results which concur with those of 

 Mr. Sharpe and Mr. Sorby already referred to, in regard to the proof of 

 pressure in a direction perpendicular to the cleavage plane; they, however, 

 for the most part, do not indicate greater relative extension on the line of 

 cleavage dip than on the line of cleavage strikef. 



On the Stratigraphical Distribution of the Oolitic Echinodermata. 

 By Thomas Wright, M.D., F.R.S.E. 



[A communication ordered to be printed entire among the Reports.] 



All the classes of the animal kingdom, when viewed in relation to their 

 stratigraphical distribution, are not of the same value to the palaeontologist. 

 Some Mollusca, as the Conchifera and Gasteropoda, have a much greater 

 extension in time than the Cephalopoda, and among the Radiata, Corals and 

 Echinoderms may be adduced as examples of classes whose species had a 

 limited life in time; in estimating the value of palaeontological evidence, it is 

 therefore necessary to take into consideration this important fact, which has 

 not received the attention it is so justly entitled to. 



The Echinodermata, although occupying a low position in the animal 

 series, in a zoological point of view, still afford the palaeontologist most 

 important data for discussing questions relative to the distribution of species 

 in time and space, for it is well known that the Silurian, Devonian, and Car- 

 boniferous rocks are all characterized by distinct forms of Crinoidea, most of 



* Mr. John Buddie gave nie an instance of this in the High Main coal of Newcastle, in 

 1834. 

 t Phil. Mag. December 18.=)6. 



