389 



zone round the garnets. The following is an analysis of the micaceous 

 rock by Mr HOUGHTON. U) 



Si0 2 ... 54-01 



Al a 8 .. 21-87 



Fe a 3 .- 5-38 



FeO ... 6-24 



MnO ... 0-63 



CaO .. 2-13 



MgO ... 1-30 



K 2 ... 3-66 



Na 2 ... 1-00 



H 2 6 ... 4-23 



100-45 

 Sp Gr. ... 2-85 



Lake District. The alteration of the Skiddaw slate by the Skiddaw 

 granite which is exposed at the surface in Sinen Gill and the Caldew 

 Valley is thus described by Mr. WARD:< 2 > "On approaching the altered area, 

 the slate first becomes faintly spotty, the spots being of a somewhat oblong 

 or oval form, and a few crystals of chiastolite appear. Then these 

 crystals become more numerous, so as to entitle the rock to the name of 

 Chiastolite Slate. This passes into a harder, more thickly bedded, foliated 

 and massive rock, Spotted (or Andalusite) Schist ; and this again into 

 Mica Schist of a generally grey or brown colour, and occurring immediately 

 around the granite." In the chiastolite slate the crystals of chiastolite 

 are often very long in proportion to their width. They lie irregularly in 

 the slate without reference to the cleavage planes. Fossils have been 

 found in slates containing well-developed chiastolite. (See Fig. 2, 

 Plate XXXIII.) 



The spotted schist forms an inner zone. It is more distinctly crystal- 

 line than the chiastolite slate. The spots are more or less rectangular or 

 ellipsoidal in form, the longest diameters lying in the plane of foliation. 

 When examined in thin section and by transmitted light the spots are seen 

 to be lighter in colour than the rest of the slide. This is due to the fact 

 that the scales of brown mica which enter largely into the composition 

 of the rock are crowded together between the spots. Minute opaque rods 

 and grains are, however, as abundant in the spots as in other portions of 

 the slide. Under crossed nicols the spots show a definite extinction 

 and are thus seen to be largely composed of some crystalline substance 

 Mr. WARD regarded them as undeveloped (that is undeveloped so far as 

 form is concerned) crystals of chiastolite. One point is clear ; they are 

 different from the spots described by Professor ROSENBUSCH, for they are 

 certainly not due to a local heaping up of the pigment in certain places. 



The rock from the immediate neighbourhood of the junction is 

 still more coarsely crystalline and mica is very conspicuous. Under the 



(1) Q.J.G.S., Vol. XXXIV., 1878, p. 224. 



(2) Geology of the Northern Part of the Lake District. Survey Memoir, p. 9. 



