393 REPOKT--1888. 



taken for sphene. When the crystals are small they are not always easily 

 distingaished. But in the granitic rocks of Tafaru-y- hotel (123) and to 

 the soath there are highly refracting and doubly refracting crystals, 

 which are clearer in colour and more vivid between crossed Nicols, which 

 almost certainly belong to this species. It is usually associated with 

 quartz, but not always. 



Augite is essentially the mineral of the dykes and minor igneous sheets 

 in this region. Such masses occur amongst the grey gneisses of Llan- 

 gwfen (125), at the boundary of the chloritic schists of Llyn Trefwll 

 (56, 57), in the boss between Amlwch and Parys Mountain (229), and 

 amongst the volcanic group north-east of Parys Mountain (243). It is 

 generally of the colourless variety, and is well characterised by its 

 cleavage and optical properties. 



Olivine in a fairly fresh state occurs in diabases south of Bodowen 

 (127), which stratigraphical and other considerations would assign to part 

 of the volcanic group in the older series. 



Diallage is equally characteristic of the gabbro area near Four Mile 

 Bridge and on Holyhead Island, where its well-marked cleavage leaves 

 no doubt of its identification. It has not been found elsewhere. No un- 

 altered olivine has yet been found in association with these rocks. 



Enstatite occurs in at least one of the better preserved rocks of the 

 gabbro area — viz., at Ty Newydd (45). It is distinguished from the 

 ordinary pyroxenes by extinguishing parallel to the traces of the cleavage 

 planes. It may be more common amongst these rocks, but not many 

 suitable slides have been examined. 



Chrysulite occurs in veins in the gabbro area in the midst of that rock 

 and of the serpentine at the inlet on Holyhead Island, south of the Four 

 Mile Bridge. It is well characterised by its fibrous structure and low 

 polarising tints. 



Serpentine, of course, forms the bulk of the rocks in the serpentine 

 area of the Western district, and is not always developed as a pseudo- 

 morph of any particular mineral, but occurs also in veins and cracks 

 traversing calcareous and other rocks. 



Talc is considered by Professor Bonney to be the mineral which pro- 

 duces the rock quarried as 'soapstone.' It is found at Pwll Olai (54). 



Fyrites is a rare mineral in these rocks, but occurs as isolated crystals 

 in a few of the granitic rocks and in the pyroxenic rocks of the volcanic 

 group N.B. of Parys Mountain. 



Hematite also occurs in minute brilliantly scarlet crystals, sometimes 

 of hexagonal outline, in the granites of Pen-bryn-yr-Eglwys. 



Magnetite may be the black mineral in undistinguishable grains which 

 makes up the groundmass of a rock at Gwalchmai (138), and it is also 

 dispersed throughout the serpentines. 



Calcite occurs in many of the rocks in such abundance as to form an 

 essential ingredient. In some cases it is undoubtedly a filtration-pro- 

 duct, as at Nebo (248). 



Tourmaline. — A few narrow purplish blue crystals, which extinguish 

 parallel to their length without the analyser and look hexagonal, may 

 belong to this species, though the crystals are very small. They occur, 

 where we might expect them, at the edge of the granite mass where it is 

 intrusive into micaceous schists at Maengwyn (122), and also in a quartz 

 segregation area in the glaucophane schist near Castellor (171). 



Structure of the Bocks of Igneotis Hahit. — The rocks now to be described 



