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PROCEEDINGS OF THE GEOLOGICAL SOCIEEY. [DeC. 2, 



judging from their present forms, they appear shortened at least 

 twenty-five per cent, in a direction perpendicular to the strike of 

 the cleavage across the bed. 



Fig. 3. 



Similar instances are common where flat shells are found in slaty 

 beds : the large Posidonise in the black shale above the limestones 

 at the base of the culm-series of Devonshire are frequently distorted 

 in the same manner ; at the limestone quarry of Bickington, one 

 mile and a half south of Barnstaple, thousands may be seen in the 

 shale thrown off the top of the quarry, every one of which is con- 

 tracted in a direction perpendicular to the line of strike of the cleavage 

 across the bed, and which are all covered with folds parallel to that 

 line. The cleavage intersects the bedding at an angle of about 50°. 

 The case of the Trilobites in the Llandeilo flags, mentioned by Pro- 

 fessor Phillips in the passage already quoted, is analogous to these. 

 In all these cases the shells appear to have suflTered from lateral 

 pressure, which has altered their forms and produced the parallel folds 

 on their surface. 



Fig. 4. 



The same lateral pressure has produced a different effect on gibbous 

 shells, which are usually flattened in a direction perpendicular to the 

 cleavage and drawn out or squeezed out considerably in the diiection 

 of the dip of the cleavage planes. The result is seen in the Spirifer 

 disjunctus, represented in figs. 4- and 5, from one of the slaty beds at 

 South Petherwin, met by the cleavage at an angle of about 60 . 

 Figure 4>Shows the shell as it lies on the bed, its length reduced 

 about one-half, from A to B, by compression perpendicular to the 

 cleavage planes, the strike of which across the bed is shown as usual 



