THE OLDER ROCKS AT ST. DAVIDS. 11^ 



was elevated out of the horizontal position, 'and before meta- 

 morphism had taken place." That is not our view : we hold that 

 they are not metamorphosed* but injected, subsequent not only to 

 the consolidation of the rocks, but to the date of their meta- 

 morphosis. 



A thin slice made out of a piece showing actual contact of the 

 Dimetian, and one of the purplish-green bands, showed no 

 apparent ^alteration in the massive rock, while the felspars of the 

 dyke in thin long prisms were alineated nearly all in one direction, 

 the result apparently of Huid-motion along the face of the 

 containing wall. For the piece whence this slice was taken I am 

 indebted to the kindness again of Dr. Hicks. 



If we may take it then as proved that these green bands are 

 intruded, it is interesting to notice that these inclined with the dip 

 or joints, and having entered along such planes, are far more 

 numerous and persistent over the whole region than those dykes 

 of similar rock which occupy vertical fissures. They show the 



* The word metamorphic is used in so many senses that misconception may 

 easily arise from its use. Changes which ensue in iocks may, for illustration 

 be roughly grouped as (i), due to air or moisture, infiltering waters, &c. : to 

 such we conceive the superficial weathering of rocks near the surface to be 

 due, and here we place the ordinary chemical re-arrangements in the dyke 

 above noticed. (2)— Local alteration due to heated intrusive rock, "contact 

 aT;crations," — we would plead for the disuse of the term " contact meta- 

 morphosis." (3) — The alterationftin rock, to which alone I could restrict the 

 term metamorphosis, viz., that to which the crj-stalline schists and some 

 granites are due (Metapepsis of Kinahan) : its causes perhaps are hydro- 

 thermal, but the exact conditions are not universally agreed upon, so that we 

 may note its conditions as more or less indeterminate, but extending over 

 large tracts, while the action of the two former is local and limited in extent 

 and their causes more explicable. This distinction of terms has been 

 previously suggested in a similar sense by I^Ir. G. Kinahan (see Trans. 

 Edinburgh Geol. Soc, Vol. Ill, p. 59, 1877.) Since, in the expression 

 "metamorphic " rocks, this technical meaning is already conveyed, it seems to 

 cause useless ambiguity to use " metamorphism " for cases where thg 

 ordinary words, ' change/ or ' alteration,' would be more fitting. 



