292 Scientific Proceedings, Royal Dublin Society. 



strata have been shifted through lateral distances, varying from 

 the smallest limits to ten, or it may be twenty miles. 1 



These earth-movements have not been necessarily horizontal, 

 their gliding-planes being often more or less highly inclined or 

 even vertical, and judging from the sections given by the officers of 

 the Survey — in what professes to be only a preliminary, but which 

 seems an almost exhaustive report, 2 so far as the description of 

 numerous details is concerned — it appears that many of the more 

 nearly horizontal master- thrusts, with their minor branches, habitu- 

 ally tended to deviate in an upward direction, as if this was the 

 position in which least resistance was encountered to the " piling 

 up " of disrupted sections by the enormous force in action — force 

 which has sometimes ground the rocks to powder, like flour in a 

 mill. Nor have these movements been confined to one geological 

 period in particular, for we find their presence recorded in the 

 original Archeean gneiss as belonging to two consecutive systems, 

 while another subsequent and perhaps more manifest development 

 is for the present referred to as " Post Lower Silurian." 3 In many 

 cases the westerly direction of the movement appears to have been 

 determinable, and it is a common feature to find the mass of rocks 

 associated with one thrust plane sharply cut off by the lower sur- 

 face of another, or whole complexes of these thrust-riven rocks 

 assuming the condition of a magnified breccia, truncated, trans- 

 gressively invaded, and overlapped by other displaced masses, like- 

 wise impelled forward by similar movements belonging to different 

 systems into which these crust-migrations are capable of being 

 grouped. 4 



An invariable accompaniment of these crust-movements is the 

 extensive regional metamorphism and deformation of the rocks, 

 found to affect both those of aqueous and of igneous origin, " dol- 

 erites becoming changed through diorite into hornblende schist," 

 " false-bedded grits and quartzites into quartz schists," " felsite 

 into sericite or mica schist," " archsean gneiss into slaty schists," 

 and " annelide tubes in quartzite being drawn out and trans- 



1 Scores of Miles in the Alps. Heini, Lapworth, and Page's Geology, p. 112. 



2 Quarterly Jour. Geol. Soc, Lon., August, 1888. 



3 Recent -work of the Geological Survey in the Highlands, Q. J. G. S., L., 1888. 



4 Recent work, Geol. Sur. in N. W. Highlands, Q. J. G. S., L., 1888, pp. 390, 430, 

 433. 



