346 SCIENCE PROGRESS 



action of pressure, the planes of foliation being developed, like 

 those of cleavage, at right angles to the direction in which the 

 pressure is exerted." 



Sometimes, under pressure, the larger crystals in a gneiss 

 have become ground down to an ovoid form by the flow of 

 smaller and powdered constituents round them. The coarser 

 granites have been milled down, 1 as it were, and foliated 

 gneisses, or even slate-like masses, have resulted. It has been 

 asserted that development of mica along the surfaces of move- 

 ment, has set up differences of constitution in different layers, 2 

 and a certain banded effect, resembling stratification, has resulted 

 within rocks of truly igneous origin, as well as within altered 

 sediments. At the same time, the intimate commingling of rocks 

 of various kinds in the great earth-mill has constructed schists 

 and gneisses out of very diverse materials. Here and there 

 a fragment of sedimentary rock may have escaped destruction ; 

 elongated quartz-patches may similarly remain, as indications of 

 the pebbles of some old conglomerate ; but for the most part the 

 origin of such schists and gneisses has become obscured by the 

 breaking down of all their original structures. 



Such, in a very few words, is the theory of dynamometa- 

 morphism, which has been employed to explain phenomena 

 outwardly as different as the uniform cleavage of slate and 

 the intense puckering of banded gneiss. Prof. Bonney 3 has 

 consistently opposed the view that shearing by itself can 

 produce a banded structure. He has, moreover, often laid 

 stress upon the probability that sedimentation in very early 

 days was so closely accompanied by metamorphic changes that 

 the Archaean or most ancient rocks present certain distinct 

 features that were impressed upon them practically at their 

 birth. In such rocks, the foliation is held by Bonney to coincide 

 with the original stratification. High temperatures prevailed, at 

 the time of their deposition, comparatively near the surface of 

 the globe, and differences in the constitution of certain layers 

 of the sediments promoted a characteristic banding as the 

 materials crystallised throughout the mass. 



1 Cj. Lapworth, Intermediate Text-book of Geology (1899), P- I2 4> etc - 



2 Lehmann, Untersuchungen iiber die Entstehung der altkrystallinischen 

 S chief ergesteine, 1884, and other authors. 



3 E.g. Discourse on "The Foundation Stones of the Earth's Crust," British 

 Association, 1888 (also in Nature^ vol. xxxix. p. 89). 



