217 



of schistosity. The ragged plates of ilmenite are deformed in the same 

 manner, but not to the same extent. The lath-shaped plagioclase has 

 completely disappeared. Its place is taken by micro- or crypto-crystalline 

 aggregates of a perfectly colourless substance having the refraction and 

 double-refraction of quartz or felspar, minute ragged scales of a colourless 

 mineral with high double refractive power, and small prisms of a mineral 

 with high refraction, low double refraction, and straight extinction 

 (zoisite). The constituents of these aggregates are, however, so small 

 and their forms so indistinct that it is extremely difficult to identify them 

 with any degree of certainty. The occurrence of a felspathic matrix in 

 chlorite-schist associated with diorite [? epidiorite] at Dobschau, in 

 Hungary, has been described by NAGY. The felspar gave a strong soda- 





The development of secondary hornblende has not been observed 

 as yet in any considerable number of cases in the diabases of North 

 Wales. This is an interesting point, because it is a very common feature 

 in the West of England. Cases, however, do occur in North Wales-. 

 Thus at Mynydd Gader, between Cader Idris and Dolgelly, and other 

 places in the same neighbourhood, a green fibrous hornblende some- 

 times replaces the augite of the greenstones. This is probably secondary 

 and the rock in which it occurs may be termed epidiorite. 



One interesting feature, frequently observed in connection with the 

 larger intrusive sheets of diabase, is the marked contact alteration 

 which they produce both above and below. This is frequently referred 

 to by Professor RAMSAY in his description of the geology of North 

 Wales. Thus, speaking of the distinctly crystalline greenstones } 

 (diabases), he says : " Finer examples still occur on the cliffs that partly 

 bound the secluded tarns of Llyn-llagi and Llyn-adar (between three and 

 four miles east of Beddgelert), where the slate in contact with them 

 is altered into a spotted porcelanic substance, such as is sometimes termed 

 ' snake-stone.' This is frequently the case with slates that lie close to 

 the greenstones of Caernarvonshire, and in such circumstances they 

 are often quarried for honestones .... Good examples of greenstone 

 and its effects may be seen on Moel-dannogen and at Bwlch-y-gerddinen, 

 on the road from Ffestiniog to Dolwyddelan. The rock is an ordinary 

 greenstone .... and largely crystalline, and the slates, both above and 

 below the mass, are porcelanised, flaky, and speckled like ' snake stones.' 

 Some slaty included fragments are also speckled, and others, which 

 like these have been caught up and included in the greenstone during 

 its intrusion have the appearance of felspathic trap [adinole]. The slates 

 all round are highly cleaved, with the exception of the bands in the 

 immediate neighbourhood of the greenstone. This greenstone, like 

 others of its class, was intruded into and altered the slates before 

 those disturbances took place that heaved and contorted the strata into 

 their present inclined positions, and also produced the cleavage ; and 



(1) N. J. Ref., 1882, I., 236. 



