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



structure. 1 The quartzites consist of distinct grains, angular or 

 rounded, cemented together by a secondary deposit of quartz, often 

 in crystalline continuity with the original grain, but those in a quartz- 

 schist are rather more irregular and indefinite in outline, so as to give 

 the matrix a slightly streaky structure and even to suggest the 

 possibility that, like the mica, they are authigenous. The quartz-schists 

 frequently pass up into mica-schists, and in one part of the Yal 

 Piora a fairly normal example is associated with a mica-schist 

 containing many rather large staurolites (24). In this district 

 masses of very dark mica-schist are abundant, and their lower part 

 is sometimes repeatedly interbanded with a not very pure quartz- 

 schist. Garnets occur in both these rocks, but are more abundant and 

 rather larger in the mica-schist. The same garnet-bearing dark 

 mica-schist occurs at the Alp Vitgira on the Lukmanier Pass (25) as 

 well as on the Nufenen Pass .(26) and in the Binnenthal (27). The 

 dark miaa-schist in the Yal Piora apparently passes locally into calc- 

 mica-schist, and it is found in a ravine on the western side of the Val 

 Canaria, associated with a pale-coloured schist, largely composed of 

 two species of mica, and with a calc-mica-schist, locally passing into 

 a white marble. 2 In fact, these different kinds of schists may be 

 seen to pass one into another, just as sandstones, clays, and lime- 

 stones do among the ordinary sediments, so there cannot be any 

 doubt that they are the metamorphic representatives of such rocks. 



In the Alps the overlying slates and other sedimentary rocks, as a 

 rule, are readily distinguishable from the above described schists, but 

 at one or two localities, authigenous garnets and staurolites have been 

 asserted to occur with belemnites, joints of crinoids, and other fossils, 

 which prove the supposed schists to belong to the Lias (28). This 

 assertion finds a merely superficial support from field evidence and 

 breaks down wholly when tested with the microscope. The minerals 

 associated with those organisms and called garnets and staurolites 

 are neither the one nor the other, but only some silicates too impure to 

 be identified with certainty; but probably allied to dipyr and 

 a colourless chloritoid, which, under certain circumstances, form 

 with comparative ease 3 and do not indicate sufficient metamorphic 

 action to obliterate any organism, though this is generally one of the 

 first things to disappear before a marked rise of temperature. Nature 

 has, in fact, been setting traps for the unwary petrologist. 



In some localities the apparent passage from a dark crystalline 



1 The pressure-modified quartzites of the Scotch Highlands, such as those 

 of Glendhu in Sutherland, present the nearest resemblance to a true quartz - 

 schist. 



2 Descriptions of these schists, together with analyses quoted from an article 

 by Dr. Grubenmann (Mitth. Thurg. naturf. Ges., Heft viii, 1888), are given 

 in a paper on " Crystalline Schists and their Relation to Mesozoic Rocks 

 in the Lepontine Alps" (Q.J. 1890, pp. 187-236). The garnets appear to be 

 an impure representative of the alumina-lime variety. They are sometimes 

 about one-third of an inch in diameter. 



3 They had already been analysed (see Q.J.G.S. 1890, p. 233) by Fritsch 

 (Beitrage zur Geol. Karte der Schweiz, Lief, xv, p. 127) and the impossibility 

 of identifying his knoten and vrismen with garnet and staurolite implicitly 

 demonstrated. 



