Mineralogy and Geology. 



Ill 



on slaty cleavage by describing the various distorted forms of certain 

 species of shells in fissile rocks, showing that these forms depend on 

 the positions of the shells with relation to the direction of cleavage. 

 He observes that the same shells in rocks that are not fissile are not 

 thus distorted ; and on a single slab or layer the various specimens are 

 all distorted in the same direction. This observation led him to throw 

 together many species which he had before considered distinct. 



He illustrates the subject by figures of distorted forms of the Spirifer 

 giganteus and Sp. disjunctus from Tintagel and South Petherwin, copies 

 of which, reduced one-half, are here given. (We have collected to- 

 gether the several separate cuts of Mr. Sharpe for more convenient 

 comparison.) 



Figure 1 to 4, Spirifer disjunctus ; 5 to 8, Spirifer giganteus. All reduced one- 

 ™t, except fig. 8, which is reduced two-thirds. 



Fig. 1 represents the S. disjunctus of its proper form, and the following 

 are distorted shells of this species and giganteus. The lines zz mark the 

 direction of the lines of cleavage. These shells, he remarks, are usually 



dicular 



vage 



drawn out, or pressed out, in the direction of the dip of the clea 

 Planes. Figure 2 represents a specimen which so lay in the rock as 

 Jo intersect the slate layers at an angle of 60° ; it is shortened one-half 

 b y the distorting force. Figure 3 is an example of a cast lying at an 

 an gle with the slaty structure of 10° or 15° ; the force causing distor- 

 J'on has pressed together the shell on one side of the middle line and 

 lengthened it out on the other ; and at the same time the shell is corn- 

 Pressed at right angles with the cleavage. Figure 4 represents another 

 Ca st in a different position ; the large part of the shell is pressed under 

 u, e other part and concealed, and at the same time the remainder is so 

 expanded that the impressions of the hinge portion are nearly double 

 h e.r usual length, this expansion taking place "as usual in the direc- 

 l on of the dip of the cleavage." Figure 5 (a cast of Spirifer gigan- 

 %*) represents a case nearly like figure 3, in which the plane of bed- 

 d, ng of the shell made an angle of less than 5° with the cleavage ; the 



