380 MESSRS. A. V. JENNINGS AND G. J. WILLIAMS 



intense metamorphism of the surrounding rocks would be a great 

 obstacle to its acceptance, even were more direct evidence unob- 

 tainable. For any observer who has gone carefully round the 

 border of the crystalline mass there can remain no doubt as to its 

 intrusive character. [At the meeting at which this paper was read 

 we exhibited photographs clearly showing the intrusion at more 

 than one point.] 



In the neighbourhood of Dolwen on the east side and of Foel 

 (near Tan-y-grisiau) on the west, not only are the Tremadoc rocks 

 in contact with it broken and twisted into sharp contortions, but 

 veins of the molten rock have penetrated the strata in all directions, 

 carrying up detached fragments along with them. Lying between 

 the granite and Cocd-y-Cribau is a small hill at Cymerau Uchaf, 

 which is composed of displaced Tremadoc rocks now in a vertical 

 position : while near Ccfn Bychan there is a vein of the intrusive 

 rock breaking through higher slate-beds at a distance of -^ mile from 

 the main mass. 



Hound the junctions the sedimentary rock is altered into a com- 

 ])act liornfels, and in some places it is very difficult to distinguish 

 the altered rock from the finely crystalline edge of the granite. 



The rock itself varies from grey to reddish-brown, and is remark- 

 able in all parts for the abundance of quartz, which causes weathered 

 surfaces to appear like those of a grit. This and the felspar, which 

 becomes red on decomposition, are the chief constituents, so that in 

 some parts the name of " aplite " might be used. Grains of darker 

 mineral are, however, usually present, and segregation -patches of 

 these are very common. Narrow veins of a dark mineral frequent!)'- 

 occur, and are often numerous and roughly parallel, giving to small 

 fragments a somewhat gneissic appearance. In cavities there occur 

 crystals of chlorite. 



Under the microscope the most noticeable feature is the appearance 

 of crush and strain seen in the broken and granular quartz. It 

 is this appearance that has suggested the comparison of the rock 

 with that of Bryn-j'-garn *, near St. David's. The quartz occurs in 

 irregular grains, often intergrown with the felspar so as to form a 

 distinct " graphic " structure. In many cases it surrounds the other 

 minerals and is evidently secondary. 



Ttie felspar is very much altered, but the cleavage is usually 

 distinct. Twinning, when shown between crossed nicols, is com- 

 monly repeated ; it is, therefore, probable that the felspathic con- 

 stituent of the rock is oligoclase, wdth perhaps a certain proportion 

 of soda-orthoclase. 



The dark minerals prove to be biotite and chlorite. The thin 

 black-looking veins consist of a brown biotite, evidently secondary, 

 with a strong fibrous appearance. 



Chlorite is common in grains throughout, and in cavities often 

 forms large crystals (hexagonal prisms with basal plane). 



[n a description of the great eurite of Cader Idris, it has been 



* Teall, ' British Petrography,' p. 319. 



