part 3] LOWER PALJEOZOIC OF ARTHOG-DOLGELLEY. 321 



Plate XVII. 



View looking eastwards across Llynau Crogenen to Pared-y-Cefn Hir. 

 ■a. Scarp made by the ' China-stone ' Ashes, b. Bryn Brith. c. Pared-y- 

 Cefn Hir, andesitic ashes, with granophyre on the steep dip-slope. 

 (7. Hollow on granophyre-ontcrop ; on the extreme right in the middle 

 distance is Mynydd Caer-go, the scarp of the Lower Basic Series. 

 Gelli-llwyd is seen in the background on the right. In the foreground 

 are grass-covered Bifidus (Crogenen) Slates with diabase-sills on 

 each side. The Bifidus Slates run along the wall leading from the 

 lake under the scarp of Cefn Hir. (See p. 279.) 



Plate XVIII. 



Pig. 1. Granophyre [C 200]. Pen Moelyn. Crogenen intrusion. Acid grano- 

 phyre of Crogenen type, granophyric structures moderately well 

 developed ; microporphyritic albite with rim of orthoclase. Crossed 

 nicols. X 20. (See pp. 291, 294.) 

 'Z. Granophyre [C 203], Hafod-dywyll, Gwynant Valley. Crogenen 

 intrusion. Similar to the above ; rounded patches consisting mainly 

 of interlocking grains of quartz with a small amount of felspar 

 Cocellar structure) surrounded by fringes of extremely fine-grained 

 micropegmatite, the whole set in a microgranitic ground-mass. 

 Crossed nicols. X 20. (See pp. 291, 294.) 



3. Spherulitic granophyre [C 206]. Nant-y-Gwyrddail. Southern margin 



of Crogenen intrusion. Spherical patches of radiate fine-grained 

 micropegmatite, almost free from dark minerals ; the spherulites are 

 set in a finely granular ground-mass richer in dark minerals. 

 Crossed nicols. X 20. (See p. 294.) 



4. Coarse granophyre, near markfieldite [C208]. Tyn-y-llwyn intrusion. 



Coarse-grained, with granophyric structures well developed. Crossed 

 nicols. X 20. (See pp. 294, 300.) 



5. Markfieldite [C 240]. Craig - y - Castell intrusion. Coarse-grained 



granophyric rock showing idiomorphic albites, granular quartz, and 

 isotropic chlorite set in a micropegmatitic ground-mass. Crossed 

 nicols. X 20. (See pp. 300, 304.) 



6. Markfieldite, near Waen-Fechan type [C 238]. Smaller Tyn-y-llwyn 



intrusion. Highly idiomorphic albites, granular quartz, with chlorite, 

 pyrites, and ilmenite, set in a ground-mass of extremely-fine, almost 

 ultramicroscopic micropegmatite. Crossed nicols, X 20. (See 

 pp. 298, 300.) 



Plate XIX. 



Fig. 1. Basic rock [C 234]. Quarry, western end of Waen-Fechan intrusion. 

 The rock is comparable with a diabase. Texture originally sub- 

 ophitic. Idiomorphic albites with interstitial chlorite (grey), large 

 plates of ilmenite, abundant wisps of biotite, and occasional blebs of 

 quartz. Ordinary light. X 20. (See pp. 289, 297.) 



2. Basic marginal rock [C 220]. Lower or northern margin of Crogenen 



intrusion, 600 yards east-north-east of Waen Fechan. Felspar 

 mainly orthoclase. Abundant dark minerals include chlorite as 

 wisps and fibres, and some ilmenite as plates. Quartz rare. Near 

 the lower margin occurs a twinned crystal of allanite, associated with 

 calcite and quartz. Ordinary light. X 20. (See p. 295.) 



3. Basic marginal rock [C 223]. Lower margin of Crogenen intrusion, 



stream near Waen Fechan. Microporphyritic albite with outgrowth 

 (above) of fine-grained micropegmatite. Dark minerals abundant ; 

 strongly pleochroic hornblende and brown biotite in wisps and fibres. 

 Ordinary light. X 20. (See p. 295.) 



