MONIIN SYSTEM OF EOCKS. 



481 



Fig. 4. — Promontory east of Forth Rhyffydd^ showing Cleavage 

 and Contortion. 



•S.E. N.N.W. 



The Mass of " Serpentine." — Much is already known about this 

 from the researches of Prof. Bonney, published in vol. xxxvii. of 

 the Journal of the Society, and it does not concern the present 

 memoir to discuss its general points of interest. The name applied 

 to the rocks which are coloured as " Serpentine " on the Survey 

 Map must be taken in a very general sense as equivalent to " some- 

 thing peculiar, related in some way to the presence of serpentine," 

 and even then the mapping is not quite correct*. From the mode 

 of occurrence of the rock, and from the teaching of the junction 

 mentioned by Prof. Bonney, it is certain we are here dealing with 

 intrusive masses. The actual masses of igneous rock are more 

 sporadic and isolated than marked on the map. In particular the 

 gabbro is confined to upstanding bosses, which have the aspect of 

 necks, and much of it is foliated on the large scale, though the 

 diallage crystals are sufficiently irregularly distributed in its sub- 

 stance. These bosses are found at Ty newydd, Morfa-coeg near 

 Cruglas, Melin Carnau., Dinas Fawr in two places, and at Ceryg- 

 moelion. To the north this rock is drawn out into schist by dynamic 

 agencies, and is not easily recognizable. The lower eminences are 

 almost entirely serpentine, and this seems almost always to be inter- 

 polated between the gabbro and the schist. There are limestones 

 also at Cruglas, behind Ceryg-moelion, and in the inlet west of Dinas 

 Fawr ; and these, when they occur, are always between the serpen- 

 tine and the schist. The largest mass of true serpentine stretches 

 southward from round the above-named inlet as far as the Roscolyn 

 road. It would be rather remarkable if the serpentine and the 

 gabbro were entirely independent, but they may only be related as 

 outbursts from a common focus. In only one place is the serpentine 

 of crystalline aspect, as in the Lizard, and in this the positions of the 

 original olivine crystals are easily discerned. The remainder is 



* There are, howeyer, some green -coloured boundaries marked within the 

 serpentine, which show that more than one kind of rock was recognized. 



2k2 



