656 FOURTH REPORT — 1834. 



stals on t^e walls to a solid mass of fibrous or lamellar carbonate 

 of lime occupying the whole cavity, and occasionally interspersed 

 with metallic substances. 



In passing through different sorts of rocks, the fissures pre- 

 serve nearly the same inclination from the vertical, but their 

 breadth and regularity are very unequal. The same fissure 

 M'hich in limestone is open, or filled with spar to some inches' 

 width, may be reduced to a mere divisional plane in the alter- 

 nating shales : the fissures in coarse sandstone are irregular, 

 those in fine shale possessed of almost crystalline symmetry. ; 



Viewed upon a horizontal plane, the fissures and joints almost 

 always, except in greatly deranged masses of rocks, appear to 

 follow definite directions ; so as by their mutual intersections 

 to dissect the area into compartments, of which the figure varies 

 according to the nature of the rock. This variation appears 

 principally due to the joints, for the great lines of fissure hold 

 nearly the same courses throughout. The joints usually ter- 

 minate in the fissures ; these last are of unequal extent, some 

 being more persistent than others, so as to deserve the title of 

 master fissures. ,.i 



In slate rocks of the North of England there are often several, 

 sets of joints, besides the cleavage, which are more or less di- 

 stinct and continuous; some of them vertical, others inclined at 

 considerable angles, and passing in different directions, remind- 

 ing us of the various intersections of the Cornish veins. In the 

 Craven slates the most constant of all the divisional planes is 

 that of cleavage which ranges W.N.W. and E.S.E., having a 

 variable dip to the S.S.W. or N.N.E. ; it is crossed by vertical 

 joints ranging nearly N. and S. 



In the mountain limestone tracts also, the great fissures are. 

 in general parallel to one or other of two lines: they either, 

 range nearly E. and W., or W. of N. and E. of S. ; of these 

 directions the latter is perhaps the most predominant. Fissures 

 in other directions are indeed often noticed, but they are of less 

 importance. 



In the magnesian limestone of Yorkshire the dii'ectio.ns of 

 the principal and most continuous fissures are from E. to W. 

 and from N. to S. : the same is the case with some parts of the 

 oolitic ranges on the north of the Vale of Pickering. 



The author concludes, from actual observation, that throughout 

 the greatest part of Yorkshire north of the Aire and Wharfe, 

 the most continuous fissures are nearly veilical, and range either 

 E. and W. or nearly so, or a little W. of N. and E. of S. 

 The tract of country thus defined connects itself on the north 

 with the mountain limestone districts of Durham, Cumber- 



