419 



A massive igneous rock after it has been deformed by the earth- 

 stresses possesses in general a more or less foliated structure which is 

 easily recognizable in hand specimens. The darker and lighter con- 

 stituents instead of being distributed uniformly through the mass are 

 now arranged in wavy or parallel layers (see Plates XXVI. and XLIIL). 

 The character of the parallel structure will depend upon the character 

 of the deformation. If the ellipsoid of deformation be such that one axis 

 is much longer than either of the other two then a kind of linear 

 parallel structure will be developed ; if, on the other hand, one axis be 

 very much shorter than the other two a kind of plane parallel structure 

 will be developed. If the original rock be porphyritic the resulting 

 foliated rock will show " eye-structure." Such a structure, however, 

 does not necessarily indicate that the original rock was porphyritic. 

 If the deformation be comparatively slight, then, in the case of such 

 rocks as coarse-grained granite and gabbro, the foliation will be of 

 the flaser-type. If it be very great a more even foliation will result. 

 Speaking broadly, we may say that the structure of typical schists is 

 indicative of great deformation. The structures here referred to are 

 due to the kind of deformation and not to the original composition of 

 the rock. They are similar in rocks of such different composition as 

 granite and gabbro (see Plate XLIIL). Thus we have granite, rlaser- 

 granite (gneiss), augen-granite (augen-gneiss), and mica-schist ; gabbro 

 flaser-gabbro, augen-gabbro and hornblende-schist. 



Although a parallel structure is a very common feature in rocks 

 that have been subjected to dynamic metamorphism it is not invari- 

 ably present. Thus BROGGER (I) has shown that near Christiania the 

 gneissose rocks pass into massive brecciated rocks in the neighbourhood 

 of the great dislocation planes which correspond with the borders of 

 the fjord. 



A rock, which has been subjected to deformation subsequent to 

 its consolidation, may consist wholly of original minerals, that is, of 

 minerals formed before the deformation, and of the mechanical detritus 

 of such minerals ; or it may consist wholly of minerals formed during 

 or subsequent to the deformation; or lastly, it may consist partly of 

 original minerals and the detritus of such minerals and partly of 

 secondary minerals. The micro-structure of the rock will be deter- 

 mined by the relations of the different constituents. In many cases 

 there is, of course, a close relation between the macro- and micro- 

 structures. The finer-grained rocks often show precisely the same 

 structural characters under the microscope that the coarser-grained 

 rocks do when viewed with the unaided eye. 



The effects of mechanical force on the individual constituents are seen 

 in the distortion and fracture of the minerals, and in the development of 

 secondary structures, such as twin-lamellation and a kind of microcline- 

 structure in the felspars. The fact that a mineral is in a state of strain 



(1) Ueber die Bildungsgeschichte des Kristianiafjords. Nyt. Mag. Band XXX. 

 Heft. 2, 1886. 



