192 SCIENCE PROGRESS. 



smaller, less perfect, and less fresh than in the Japanese 

 occurrence. Hutchings (9) has recently identified as cor- 

 dierite the essential mineral of the "spots" in the metamor- 

 phosed Coniston Flags near the Shap granite, and remarks 

 its close resemblance to the occurrence noted by Beck in the 

 Elbe Valley in Saxony. The grains are small, rounded, 

 and full of inclusions, and do not show the polysynthetic 

 twinning so characteristic of better developed crystals of cor- 

 dierite. In view of this discovery and that near the Skid- 

 daw granite, it seems probable that cordierite may be found 

 in other areas of "spotted" slates in this country. 



The curious spotted or nodular rocks (Fleckschiefer, 

 Knotenschiefer, etc.), which are so common in districts of 

 metamorphosed slates, still present some difficult problems. 

 In one type the spots, as described by Rosenbusch and 

 others, are due merely to local aggregation of the carbon- 

 aceous or other pigment which was originally disseminated 

 through the general mass. Rocks of this kind are found in 

 the outer border of many areas of thermal metamorphism, 

 and usually show r little or no important recrystallisation. 

 Quite distinct from these are spotted rocks showing more 

 advanced alteration, in which the spots are little patches 

 relatively free from the biotite or other conspicuous second- 

 ary mineral produced in the metamorphism. Examined 

 by optical tests in thin slices, these spots often show a dis- 

 tinct crystalline reaction, and are found to be really im- 

 perfectly formed crystals, often further obscured by numerous 

 foreign inclusions. In some cases the constituent mineral 

 of the spots has been found to be andalusite ; in others, as 

 already mentioned, it is cordierite ; and it is possible that 

 other aluminous silicates may figure in some other cases. 

 It is to be noticed that in the most advanced stages of meta- 

 morphism spotted and nodular structures of all kinds have 

 usually disappeared altogether, and the same is often true 

 of the more marked foliation and parallel-structures fre- 

 quently observed in the less metamorphosed strata. The 

 rocks at the inner border of an aureole of contact meta- 

 morphism (the Hornfels of German writers) commonly 

 .show in thin slices a mosaic of irregular polygonal grains 



