RELATION TO MESOZOIC ROCKS IN THE LEPONTINE ALPS. 



225 



mica. We find also, in some cases rather abundantl}', a colourless 

 mineral in moderately elongated prisms (ranging in length from 

 about -02 inch to -06 inch). These, with crossed nicols, give 

 apparently a straight extinction and low bluish tints in intermediate 

 positions. They occur both when garnet is present and when it is 

 absent. Dr. Grubenmann identifies this mineral with zoisite, and 

 suggests the possibility of its having replaced garnet ; but, so far as 

 I have seen, it appears to me to have been formed independently, and 

 to belong to the same period in the history of the rock. In certain 

 cases, I note among these prisms a slight difference in aspect and 

 some obliquity in extinction, so that kyanite may also occur. 

 Tourmaline is present, being sometimes fairly abundant. Some of 

 it, however, is rather peculiar in aspect, for it occurs not only in 

 more or less definite prisms, but also in films. In the latter case it 

 may have replaced brown mica, as it often appears to do in granite. 

 The colour is a dull indigo or slightly greenish blue. Small prisms, 

 and occasionally geniculate twins, of a honey-brown mineral, are not 

 unfrequent ; these are probably rutile. Sometimes they are locally 

 very abundant. 1 have counted more than 30 in a field, measuring 

 •04 inch in diameter*. Granules of magnetite, haematite, and 

 pyrite are present. Every slide that I have examined of this rock 

 indicates a crushing more or less severe, with probably a slight 

 shearii]g movement. To this also I am disposed to attribute the 

 brownish dust present in the slide, instead of regarding it as an 

 original constituent. No one, I think, accustomed to the study of 

 crystalline schists and of the effects of pressure can have the 

 slightest doubt that the rock was a crystalline schist, containing 

 sometimes good-sized garnets, anterior to its being exposed to the 

 only pressure of which it has retained a definite record t. 



* The following analysis of the Grcmaffiihrcndc Thonglimraer-Schiefer has 

 been made by Dr. Grubenmann. As rock analyses always have great value, and 

 his paper appears in a publication not very accessible to English readers, I 

 shall not hesitate to reprint them here, lie states that the garnets were too 

 impure to give a satisfactory result, but that a partial analysis showed them to 

 be an alumina-lime garnet (thus corresponding with those in the Tremola 

 schists). The large amount of TiO.^ shows that much rutile may be expected 

 in the rock • — 



SiO., T2"l-i 



TiO, 301 



Al.,0, 11'7'J 



Vefi,, 209 



FeO 1-74 „ o K^p- 



CaO 40(; ^1^- g*"- =3-560/. 



MgO 0-80 



K,0 0-85 



Na.,0 007 



Combustion-loss (H^O, CO,,) 2-30 



100-7.5 



t See also President's Address, 1885, Quart. Journ. Gool. Soc. vol. xli.. Proc. 

 pp. 40, 47. No specimen in my collection gives clearer evidence in proof of the 

 above statement tliau that which I obtained from the ravine (Val Canaria) 

 which was examined by Dr. Grubenmann. 



