200 Prof. T. 0. Bonney — Foliation and Metamorphism. 



escarpment, often very conspicuous, which runs from the southern 

 end of Loch Maree to the north of Loch Eriboll, the members of 

 which are generally "slabby" in structure, composed mainly of 

 quartz, felspar, and mica, though the minute scales of the last appear 

 to have been formed in situ. 1 This group of gneisses was asserted to 

 represent a set of metamorphosed stratified rocks, approximately of 

 Silurian age. Difficulties in this interpretation became graver as 

 the knowledge of petrology increased, till, in the summer of 1882, 

 the problem was finally solved by Professor Lapworth (12), who 

 demonstrated that some of the pre-Torridonian crystalline rocks had 

 been carried by .overfolding and overthrusting above sedimentaries 

 of later date, and had been greatly crushed in the process. So these 

 "Newer Gneisses" are really Archsean Gneisses, which have under- 

 gone pressure-modification, a clastic structure (followed by a certain 

 amount of mineral development, as described above) having been 

 imposed on them at some epoch subsequent to the latest Cambrian and 

 distinctly before the Devonian periods (13). A similar origin was 

 claimed by Professor Lehmann in 1884 (14) for certain slabby gneisses 

 in Saxony, but in this region the gneiss appears to have been more 

 completely reconstituted, so perhaps the crushing may have occurred 

 at an earlier date, or some local cause have favoured the repairing 

 process. 



Rocks of a dioritic or doleritic nature also exhibit a pressure- 

 foliation. The felspathic and ferro-magnesian constituents are 

 crushed, and to a certain extent "rolled out", after which they 

 undergo a process of reconstitution, analogous with that which has 

 been described in the case of granite rocks, but the original augite or 

 hornblende is replaced by an acicular and generally rather smaller 

 form of the latter mineral. The rock becomes, in fact, an actinolitic 

 schist, examples of which are common in the Alps (15). The more 

 coarsely crystalline hornblendic gneisses, such as occur in the 

 Lepontine Alps for some distance eastward from the south side of 

 the St. Gotthard Pass (16) and in one or two parts of the Tyrol, also 

 afford examples of a coarser form of pressure-foliation. 2 



The bulk of gneisses, especially the banded kinds, were formerly 

 supposed to be metamorphosed sedimentary rocks. That was con- 

 fidently stated less than half a century ago in textbooks of authority 

 (17), and I well remember how often sedimentary rocks were said to 

 pass (i.e. be melted down in situ) into igneous rock (18) ; nay, 

 a metamorphic origin was sometimes attributed even to granite, 

 because, as the specific gravity of its quartz was 2 - 65, and that of 

 quartz which had been melted was only 2"25, the rock could not have 

 been truly fused. 3 It is now not too much to say that the onus 



1 Similar gneissoid rocks occur on the southern border of the Highland 

 complex from near Stonehaven northward for some miles. 



2 Certain minerals, such as hornblende, biotite, chloritoid, ottrelite, dipyr, 

 couseranite, and a plagioclase felspar, seem to form with comparative ease, as 

 will presently be described, in some crushed gneisses and hornblendic rocks. 



3 The advocates of this notion appear to have forgotten that normal 

 crystalline quartz is a frequent constituent of felsites, rhyolites, etc. 



