B18 



it is clear that the altered porcelanic character of the rocks in contact 

 with the greenstone prevented their being cleaved."* 1 ' 



Again, referring to the slates altered by greenstone in the neigh- 

 bourhood of Llyn Cwlyd, the same author says: "These porcelanites 

 weather white, like the felspathic rocks, and I found it no easy matter 

 to separate the two while mapping the country between Pen-uchaf-y- 

 Gwaith, Llyn Cawlyd, and Capel Curig." Good illustrations of the 

 contact metamorphism of diabase may be studied in the neighbourhood 

 of Tremadoc. The great mass of intrusive rock which forms Y.-Gesell 

 has markedly affected the strata with which it comes in contact. (3) The 

 dominant type of metamorphic rock is a light, grey, speckled mass often 

 showing marked schistosity. The speckling is due to minute spots 

 readily distinguishable with a hand lens. These spots lie in a light 

 coloured ground-mass. In a thin section they are much less marked 

 when viewed with ordinary light, than they are in the rock-mass. 

 Under crossed nicols, however, they become apparent, owing to the fact 

 that they are much darker in all positions of the stage than the general 

 mass of the rock. The small size of the individual constituents renders 

 it impossible to define the composition of this rock with any degree oi 

 precision. Scales of chlorite lying with their flat surfaces parallel Avith 

 the planes ot schistosity can be distinctly recognized ; also scales of a 

 more vividly polarizing micaceous mineral, probably sericite, aggregates 

 of irregular grams having the refraction and double refraction of quartz 

 and felspar. The vividly polarizing mica is much less abundant in 

 the spots than in the general mass of the rock. The darkness of the 

 spots under crossed nicols seems to suggest the presence of an isotropic 

 substance. This rock answers to the spilosite of ZIXCKEX, and the Hartz 

 geologists/ 4 * 



A typical spilosite, according to LOSSEX, consists of : 



(1) An isotropic substance forming a kind of base in which the other 



constituents are embedded. 



(2) Very numerous and extremely minute scales of a micaceous mineral 



which, together with the isotropic substance, make up the main 

 mass of the rock. These lie for the most part with then* broad 

 surfaces parallel \vith the lamination of the rock. 



(3) Small aggregates of yelloAvish green scales of less transparency and 



loAver polarization tints than the mica. These resemble rudiment ary 

 spots. 



(4) Flecks of a dark grey cloudy substance. This probably represents 



(1) The Geology of North Wales. Geol. Survey Memoirs, Vol. III., Second Edition 

 1881), p. 125. 



(2) Loe. cit., p. 135. 



(3) The metamorphic band has been mapped as a grit in the Survey Map of the district. 



(4) Uber den Spilosit und Desmoisit Zincken's. K. A. LOSSES, Z. D. G. G., Band XXIV., 

 1872, p. 701. See also KATSKE, Uber die Contactmetamorphose der kornigen Diabase im Hartz, 

 Z. D. G. G., Band XXII. (1870), p. 103; and VON LASAOTX, P. A., Band CXLVIL, Stuck 1, 

 Heft 9, p. 141, Heft 10, p. 283. 



