106 Pruf. Hughes, Criticism of the Geological [Oct. 30, 



breccia has been formed and minerals of different hardness have 

 crystallized along the fissure. A good example of this is seen in 

 No. 42, which is a piece of fault breccia which has been itself 

 faulted and the included fragments and the matrix have been 

 scored by the passage over them of harder fragments in the 

 opposing face. We may find produced in this way many of the 

 varieties which we observed in the case of true glacial striae. 

 The rock may be fluted but not polished as in No. 48, which is a 

 fragment from a fault face in Silurian rocks of the Vale of Clwyd, 

 or we may find a coarse grit first deeply grooved and polished and 

 then covered with a glistening mineral film, as in the specimen 

 No. 44 from Nodale, Stansfield Moor, or in the brighter surface 

 of No. 45 from Derbyshire. Sometimes we find evidence of 

 change of direction producing secondary striation oblique to the 

 first, or more rarely, as on the specimen from a quarry north of 

 Aberystwith, No. 46, we may see from the curved lines on the 

 mineral layer and on the rock that the direction of movement has 

 changed. 



A fluted surface is produced in some rocks by a combination 

 of thin bands of tougher rock with joints and cleavage so disposed 

 that the unyielding bands, alternating with the portions of the 

 rock which are compressible and susceptible of cleavage, are pro- 

 truded or displaced at the planes of interruption caused by the 

 joints and thus present parallel ridges, as seen in the specimen of 

 Silurian rock. No. 47, from Tirgwyn in the Vale of Clwyd. 



It often happens that when a rock, with fine lines of sediment 

 that weather out more readily, is exposed to denuding agents, the 

 occurrence of those lines of weakness is indicated by finer or 

 coarser parallel grooves, and if such a rock lies in boulder clay, 

 the strise may very likely be all referred to glacial action. 

 Examples of this may often be found among Silurian and Cam- 

 brian rocks, as for instance on the specimen No. 48 from the 

 drift of Maes Mynan near Caerwys. Sometimes such lines and 

 grooves indicate joints which are otherwise quite invisible on the 

 face of the rock. This is well seen on the specimen No. 49 of 

 sandy shale, probably Devonian, from the slopes of Dartmoor, on 

 which the divisional planes which have caused the grooves on the 

 face of the stone are seen as joints on the side. 



Jf a conglomerate be subjected to pressure of the same kind as 

 that which produces crumpling or cleavage in one place, faults in 

 another, or belts of puckered rock in a third, it often happens that 

 the rock, yielding along definite areas, readjusts itself by move- 

 ment between the portions of different tenacity and a shearing 

 takes place throughout, with distinct thrusts and displacements 

 between the matrix and the included pebbles, see Nos. 50, 51, 

 while smaller fragments of greater hardness imbedded in the 



