CHANGES IN CEETAIN ERUPTIVE ROCKS OP NORTH WALES. 429 



It is evident that the crystals under consideration differ essentially 

 in their origin from those which, while possessing the external form 

 of augite <fcc, are, to a large extent, composed of magnetite or ilme- 

 nite ; the latter exhibit sharply defined outlines, and, as suggested 

 by Zirkel, were probably formed under conditions similar to those 

 which resulted in the crystallization of the quartziferous limestones 

 of Fontainebleau*. The alteration of the crystals of uralite, on the 

 contrary, begins externally and extends gradually towards the 

 centre. 



Among the more indistinct pseudomorphs observed in the sec- 

 tions examined are a few filled with a granular transparent material, 

 containing quartz granules and divided by reticulated lines of minute 

 crystalline aggregations in a way suggestive of the fissures so gene- 

 rally observed in olivine. It may be remarked that this rock is 

 almost identical in chemical composition with the ancient dolerites 

 from the neighbourhood of Penzance, and also that recent observa- 

 tions have shown that the augite in certain Cornish dolerites has 

 become partially transformed into uralite. 



EXPLANATION OF PLATE XIX. 



(All the figures magnified 17 diameters.) 



Fig. 1. Twin crystal of uralite which has undergone alteration around its 

 edges and, to a less extent, along a line of fissure by which it is 

 divided. 



Fig. 2. Crystal of uralite, altered to a greater depth than in the former case. 



Fig. 3. Uralite ci-ystal which has been entirely removed and replaced by 

 various secondary products. 



Discussion. 



Mr. Warington W. Smyth stated that he had examined the 

 quarries referred to, but without arriving at such satisfactory results 

 as those brought forward by the author, to whom he felt obliged 

 for several important suggestions. The rock appeared to be mainly 

 all of one variety, though some geologists might perhaps be inclined 

 to subdivide this great mountain-mass. Chemically and commer- 

 cially, indeed, there is a great difference between the several parts 

 of this mountain. At the western extremity some little variation 

 occurs ; but at the eastern end, at Graig-lwyd, there were great dif- 

 ferences. Great masses are left unworked from year to year because 

 they do not wear well. These are of a finer grain, and there is 

 no appearance of free silica in them, although it may exist in the 

 other rock ; and Mr. Phillips has shown that when free silica is 

 present the rock is more useful. The discovery of the peculiar 

 jointing which enables the rock to be easily cut into " sets," has led 

 to its being largely worked; and there is a great exportation of it 

 to Manchester, Liverpool, &c. The discovery of free silica in the 



* Untersuchungen iiber die niikroskopische Zusammensetzung und Structur 

 der Basaltgesteine, p. 27. Bonn : 1870. 



