CHANGES IN CERTAIN ERUPTIVE ROCKS OE NORTH WALES. 427 



I. II. 



Wnter f h yg r ometric -73 -73 



Warer 1 combined 621 630 



Silica 34-49 3484 



Phosphoric anhydride trace trace 



Alumina 21-78 21*65 



Ferric oxide 19*48 19-31 



persulphide -69 *69 



Ferrous oxide trace trace 



Manganous oxide 10*26 10-35 



Lime 316 3*12 



Magnesia 1*75 1-60 



Potassa .., trace trace 



Soda 119 1-24 



99-74 99-33 



The presence of a notable amount of manganese in a somewhat 

 similar deposit, has been observed by Von Lasaulx, who found 4-58 

 per cent, of manganic oxide in a volcanic ash-bed in Auvergne *. 

 In the Penmaenmawr rock the oxygen present is just sufficient to 

 form ferric and manganous oxides ; in such cases it is impossible to 

 determine the state of oxidation in which the two metals respec- 

 tively occur. 



The composition of this deposit differs very considerably from 

 that of the neighbouring crystalline rock, as, in addition to a con- 

 siderable amount of oxide of manganese, there is a large diminution 

 in the percentage of silica. 



Senarmont and Daubree have shown that water at high tempera- 

 tures and under great pressure is capable of attacking silicates 

 and of removing from them a portion of their silica ; it therefore 

 appears not improbable that this deposit may be the result of a 

 flow of volcanic mud from which a portion of its silica has been 

 thus removed f. Whether oxide of manganese was an original 

 constituent or has been subsequently introduced by infiltration, 

 would be difficult to determine. 



An instructive example of the changes which sometimes take 

 place in crystalline rocks, and in which their quarry-water is pro- 

 bably an important agent, is afforded by the " uralite porphyry," or 

 uralitic dolerite of the Mawddach valley near Dolgelly. 



An outcrop of this rock is seen near the summit of a hill imme- 

 diately north of the road a mile below Tyn-y-groes ; it is of 

 a greyish green colour, plentifully spotted with black, and consists 

 of a granular base enclosing numerous patches and crystals of 

 uralite, many of the latter being above half an inch in length. The 

 outlines of these crystals are occasionally sharp and well defined, 

 while in other cases they are rounded and gradually merge into the 

 general base. In addition to crystals of uralite this rock encloses 

 some whitish patches of a felsitic material, which occasionally exhibit 



* 'Roches Volcaniques de 1' Auvergne,' p. 71 (Clermont Ferrand: 1875). 

 Translated hy F. Gonnard. 



t The basic slaty hornblendic rocks of the neighbourhood of Penzance 

 (Quart. Journ. Geol. Soc. vol. xxxii. p. 165) may have had a similar origin. 



