AND ASSOCIATED HOCKS OF ANGLESEY. 



45 



the quarry south of Cruglas consists of three minerals : — (a) a clear 

 transparent mineral, with a satiny texture and occasional very faint 

 indications of two cleavage-planes ; with crossed Nicols it presents 

 a dark base crowded with irregular patches of minute fibrous micro- 

 liths of a dull yellowish colour, (b) About an equal quantity of 

 roundish or irregularly polygonal grains of a pale yellowish green 

 mineral, which exhibits one marked set of cleavage-planes, and a 

 second less perfect at right angles ; these, especially the former, are 

 further indicated by numerous grains and rods of opacite, probably 

 magnetite. With crossed Mcols this second constituent is either 

 black or shows a pale fibrous dull bluish mineral. The greatest 

 absorption takes place when the cleavage-planes are parallel to one 

 of the vibration-planes of the Nicols. (c) Scattered black grains and 

 crystals of magnetite or possibly chromite. JSFo structure resembling 

 olivine occurs in the slide ; so far as it is possible to conjecture the 

 nature of the unaltered rock, it most resembles one that has consisted 

 almost wholly either of two kinds of enstatite*, one richer than 

 the other in iron, or of enstatite and hypersthene. I much regret 

 that I could not succeed in finding the rock in situ. 



The Ty-Ucha rock, on the contrary, has the granular structure 

 (defined by wavy irregular strings) which is often seen in an altered 

 olivine rock. These, with crossed Nicols, exhibit a slightly fibrous 

 structure and bluish- white colour, with an occasional golden-yellow 

 tint, as is common in serpentines, the interstices being dark or 

 relieved by patches of dull bluish fibres. Opacite and larger grains, 

 generally rounded, but sometimes clearly crystals of the isometric 

 system, are scattered about the slide ; probably these are magnetite. 

 No appreciable quantity of any other mineral is present in the slide. 

 This serpentine then probably results from the alteration of an olivine- 

 roek, a kind of dunite. 



analyses : — I. is the dull-green chloritic rock, and II. the talcose schist. 

 No. I., however, has not made it easier to identify the chief mineral. 



Water 

 Si 0., 

 Al a 8 



p e ; o 3 



FeO 

 Cr,0.. 

 Mn 6 

 CaO 

 Mg O 

 Alkalies 



. 11-09 

 . 28-56 

 . 39-54 

 . 0-99 

 . 2-87 

 , traces. 

 . traces. 

 , T73 

 15-79 

 0-70 



101-27 



II. 



Water 2-1 



Si 2 



ai.,o; 

 F e ;o 3 



Fe O 



56-34 



8-21 



3-04 



200 



Mn O traces. 



Ca O 0-52 



Mg O 25-43 



Na.,0 0-79 



99-19 



* Prof. Maskelyne has described an enstatite rock (without olivine or other 

 mineral, except perhaps diopside) from S. Africa. Another slide cut from an 

 outcrop in this neighbourhood presents some resemblance to the above, though 

 the constituents are more minute and more highly altered, and there is less 



