122 Marker: Cordicrilc iu f/w Metamorphosed Skidihiw Slates. 



position ot" the rock in each bed. There is no difficulty in 

 distinsjfuishin^- the two in thin slices. Cordierite is inferior to 

 quartz in mean refractive index and in birefring^ence, while 

 andalusite stands well above quartz in both characters. Further, 

 the i^ood prismatic cleavage, which is usually well seen in 

 andalusite, is wanting^ in cordierite. The other common 

 minerals in these metamorphosed slates are : biotite, con- 

 stantly abundant in rai^g^ed flakes ; muscovite, less plentiful 

 and in smaller scales ; chlorite, and sometimes ilmenite, with 

 the same habit ; quartz, in the more siliceous slates only ; 

 g;raphite or opaque carbonaceous matter, almost always present ; 

 minute crystals of mag'netite ; and sometimes ijarnets of micro- 

 scopic size. In particular beds a certain mineral, such as 

 quartz or white mica, may be specially abundant, as determined 

 by the composition of the material metamorphosed. 



The crystals of cordierite are rarely more than one-eig"hth 

 inch in diameter, and often less than half that size. They elude 

 observation, however, less by their size than by their capacity 

 for enclosing" other constituents of the rock. In thin slices, 

 seen with natural Hg'ht, the outlines are, as a rule, almost 

 invisible ; and the crystals may appear merely as relatively clear 

 spaces, containing- less brown mica than the rest of the rock 

 (Plate X., fig", i). With crossed nicols it is seen that the cordierite 

 crystals are usually very imperfectly bounded. It is also seen 

 that they are often not simple crystals, but complex twins. A 

 cross-section then shows a division into six parts, representing- 

 repeated twinning^ on the prism-plane (Plate X., fig- 2). This is 

 well known in cordierite, but it seems to be unusually prevalent 

 in these rocks. The dividingf lines are not very reg-ular, and 

 there are often narrow lamelhe or wedges included in one 

 individual of the twin, which behave optically with the adjoining^ 

 individual. A long-itudinal section of such a twinned crystal 

 shows rather irreg-ular parallel or sub-parallel lamelhe in the 

 direction of the long- axis. 



Like other aluminous silicates in metamorphosed rocks 

 (andalusite, staurolite, &c.), cordierite has the property of 

 enclosing- a larg^e amount of foreig-n material. When it occurs 

 in isolated crystal-grains, it may include only small granules 

 and flakes of the other minerals, the bulk of these being 

 expelled beyond the border (Plate X., ligs. i and 2). But in many 

 of these metamorphosed Skiddaw slates, cordierite, making up 

 the greater part of the rock, constitutes a kind of grounc^ .s 



of irregular grains, luting together without interspaces The 



Naturalist, 



