266 Alfred Harker — Stages of Slaty Cleavage. 



median and anal are tbe only branched veins, and the former 

 occu2Dies almost the entire wing ; its lowest branch originates close 

 to the base of the wing (not seen in the specimen), and although it 

 runs at a wide distance from the simple internomedian vein, it is 

 only shortly before the middle of the wing that it throws off its first 

 offshoots, which part from the branch at a wider angle than else- 

 where in the wing ; for nearly all the subordinate nervules are 

 closely crowded together. The anal area extends far beyond the 

 middle of the wing, is comparatively narrow, and filled with very 

 longitudinal branches. Moderately distant cross-veins fill the wing, 

 mostly straight and transverse, but in the broader interspaces irre- 

 gular and often branching. The length of the fragment is 75 mm., 

 and its width 40 mm. The outer margin is everywhere broken by 

 reaching the end of the nodule, but the probable length of the wing 

 was 130 mm., its breadth hardly, if at all, greater than is preserved. 

 The expanse of wings of the living insect must have been somewhere 

 from 250 to 300 mm., or somewhat more than ten inches. 



The exact locality from which the specimen was obtained is not 

 known, but Mr. Higgins says that it certainly comes from the Liver- 

 jjool Coal-field. The rapidly increasing number of Carboniferous 

 winged insects in other parts of the world should stimulate search 

 in Great Britain, for the actual number of forms known to-day is 

 probably double what it was fifteen years ago. These are the first 

 Frotophasniida recorded from Great Britain. 



Y. — On the Successive Stages of Slaty Cleavage. 



By Alfred Harker, B.A., F.G.S., 



Demonstrator in Petrology in the Woodwardian Museum, Cambridge. 



SINCE a " shear " is mathematically the same as a compression in 

 one direction with a compensating expansion in a direction at 

 right angles to it, the consideration whether slaty cleavage can be 

 ascribed to movements of this character resolves itself into the 

 question whether the cleaved rocks have or have not suffered a total 

 diminution of bulk. The most convenient way of treating the 

 question is by discussing, as I did in my former paper (p. 15), the 

 form of the " ellipsoid of distortion." The ellipsoid produced by a 

 pure shear would be one of three unequal axes, of which the second 

 or mean axis would be a geometric mean between the other two : 

 Vi^hereas, if the expansion did not compensate the compression, the 

 second axis would be greater than this geometric mean, and if the 

 expansion were slight, the second and greaiest axes would be nearly 

 equal. The facts seem to accord with the latter supposition, and the 

 diminution of bulk thus indicated agrees with what might be a priori 

 expected. 



Consider, for instance, the probable behaviour of a rock composed 

 largely of fragments of long and flat forms, having initially no 

 cleavage structure, and operated upon by a continued longitudinal 

 pressure which for clearness we may imagine as operating in a 

 horizontal direction. It appears manifest that the first result of such 

 pressure would be a horizontal compression of the rock, involving 



