46 



PEOF. T. G. BOHITEY OX THE SERPEXTIXE 



The Ty-Newydd rock* is similar to the last, but contains a little 

 of a mineral which, optically and in all other resjjects, resembles a 

 rather altered enstatite. One of the black grains shows a parallel 

 cleavage, as if it were a pseudomorph of enstatite in magnetite, a 

 thing not very uncommon in the case of augite. A few strings of 

 chrysotile traverse the slide. 



The serpentine from Ceryg Moelion consists partly of a fibrous- 

 looking doubly refracting mineral akin to chrysotile, and partly of a 

 clear, rather satiny mineral, dark or faintly granulated with bluish 

 light, resembling one of those described in the block found at 

 Cruglas (a), with thickly clotted opacite in irregular strings and 

 some scattered grains. It may be an altered olivine rock. 



The rock from a small quarry about a quarter of a mile north of 

 Plas Goch appears to be mainly composed of flaky mica-like plates 

 of a doubly refracting serpentinous mineral, with the usual granules, 

 rods, and clotted strings, probably of magnetite, and a very little of 

 the mineral described above (b) in the block found at Cruglas. Of 

 all those examined, it most resembles an altered olivine rock. 



A slide cut from the tougher serpentine in the quarry south of 

 Cruglas shows a rather peculiar structure. The slide is traversed by 

 irregular strings of clotted opacite, and is composed of two closely 

 associated minerals — one the scaly serpentine described above, the 

 other exceedingly minute scales of a doubly refracting mineral, 

 dichroic, changing from pale yellowish to dull grey-brown, and with 

 the two JSTicols giving fairly brilliant colours. There is nothing 

 inconsistent with this serpentine being the result of the alteration of 

 an olivine rock ; but, so far as my experience goes, its appearance is 

 exceptional. As stated above, I could see no evidence that this 

 variety was intrusive in the normal serpentine. 



Microscopic examination confirms the view taken in the field, that 

 several of the so-called serpentines are true gabbros. The specimen 

 from the great mass near Ty Newydd shows the ordinary structure 

 of a typical gabbro, though the felspathic constituent is wholly re- 

 placed by secondary products (much being an opaque dust), being 

 thus a variety of the saussuritic mineral so common in old gabbros. 

 There is also plenty of a rather coarsely cleaved diallage, with pro- 

 bably some ordinary augite, and a few grains of a more finely 



* I am indebted for the following analysis of this serpentine to the kindness 

 of F. T. S. Houghton, Esq., F.G.S. :— 



Water 1252 



Si o 3S-62 



AL,0" 3 415 



Fe~,0 3 5-21 



Fe O 4-34 



Mn O traces. 



Ca O traces. 



Mg O 33 83 



Alkalies 0-70 



99-37 



