560 Reviews — Ramsay's Geology of North Wales. 



blende) which under the influence of pressure would develop into 

 greenstone, might, if it were poured out at the earth's surface, form 

 sometimes only a variety of felstone, the hornblendic ingredients 

 having entered into other less easily detected combinations, con- 

 sequent upon the more rapid cooling of the rock. Another 

 anomalous circumstance connected with these intrusive greenstones is 

 that they will sometimes run in the line of strike, and occupy the 

 place of some of the ashes without appearing to thicken the general 

 mass of the beds. " However strange it may appear, this circum- 

 stance almost induces the belief that these greenstones have been 

 formed by the melting up of a part of the ashes amid which they 

 lie," and which in such cases must be supposed to have contained 

 the elements of hornblende. Our space will not permit us to notice 

 more fully the truly eruj)ted rocks of North Wales. They are to be 

 referred, as already stated, chiefly to two great horizons — namely, the 

 Llandeilo and Bala periods. But there are certain narrow greenstone 

 dykes that intersect the strata in an east and west direction which are 

 believed with some probability to be of post-Carboniferous age. 



Those who carefully consider what is advanced with regard to the 

 metamorphic nature of certain crystalline masses will find in the 

 highly suggestive details many facts that strike at the root of that 

 theory which seeks to account for the origin of granite and other 

 kindred rocks by supposing them to have been intruded from below. 

 All the phenomena associated with the great Cambrian quartz-por- 

 phyry, that stretches between St. Ann's Chapel and Llanllyfin, un- 

 doubtedly go to prove that this rock has originated from the meta- 

 morphism of the aqueous strata ; and the same may be said of the 

 granite of Anglesey. Both rocks melt insensibly into the surround- 

 ing beds. The latter are not pushed aside to make room for them, 

 but have actuallj' supplied the material which, under the influence of 

 metamorphism, has crystallized into granite and porphyry. This 

 being the case, it is not surprising to find that contortions and fault- 

 ings affect both stratified and amorphous rocks alike. 



In the description given of the geology of Anglesey there is much 

 suggestive matter on the subjects of cleavage and foliation. After 

 having stated his opinion — which is also that of other geologists — 

 that foliation may follow lines of dip or false bedding, or planes of 

 cleavage according to circumstances. Prof. Kamsay suggests an in- 

 genious theory to account for the origin of contorted foliation. He 

 says, " Suppose true slaty cleavage to cut in straight lines through 

 beds inclined at any angle, and that afterwards foliation was super- 

 induced in the line of the cleavage planes ; then, while foliation was 

 being produced by a chemical re-arrangement of matter accompanied 

 by moisture and heat, the expansion produced by heat at great 

 depths would induce intense pressure, one result of which might be 

 that such pressure, if exerted vertically from below, would in some 

 instances compress the beds and reduce their individual thickness 

 without obliterating the stratification, while the foliated cleavage 

 planes, also vertically compressed, might lie forced into contorted lines 

 more or less across the planes of bedding." The theory is further ex- 

 plained by means of a diagram. 



