338 



the Thuringerwald. (1) In describing the basic igneous rocks of North 

 Wales it was shown that they locally become schistose and sometimes 

 pass into a chloritic schist In the same way the acid rocks locally 

 become schistose and sometimes pass into sericitic schists. Now, seeing 

 that devitrification is accompanied by an increase in density it is 

 reasonable to suppose that intense mechanical stresses would tend to 

 bring about devitrification. The devitrification of the Ordovician obsi- 

 dians and pitchstones is therefore possibly a phenomenon of dynamic 

 metamorphism. 



The nodular character of many of the felsites of North Wales 

 is a somewhat striking feature. The nodular felsites have especially 

 engaged the attention of Dr. BONNE Y and Mr. COLE. The " nodules " lie 

 in a, felsitic matrix in which fluxion structure is often well-defined. 

 They may be solid or hollow. In the latter case the cavities are often 

 lined with quartz crystals. Sometimes the cavities have been entirely 

 filled with quartz. In the cases examined by Dr. BONNE Y the 

 material of the nodules was similar to that of the felsite. There was 

 no trace of radial structure. Mr. COLE records the occasional presence 

 of radial structure. The two observers above-mentioned differ as to 

 the origin of the cavities. According to the former they are original, 

 according to the latter they have been formed by the decomposition 

 of solid spherulites. That they are analogous to the " lithophysse " of 

 VON RICHTOFEN will be admitted, and it is interesting to observe that 

 the same difference of opinion exists with regard to the origin of these 

 structures. A discussion of the question so far as it affects the origin 

 of the liihophysrc of vitreous rocks will be found in a paper by Mr. 

 J. P. IDDINGS. < 2) This author concludes that the lithophysa^ in the 

 obsidian of the Yellowstone region, containing quartz, tridymite, acid 

 felspar (soda-orthoclase) and fayalite, are due to the liberation of gases 

 and vapours accompanying the crystallization of the spherulites. If 

 this view be generally applicable then hollow spaces should occur only 

 in those spherulites which are crystalline in texture. It will not 

 explain the occurrence of hollow spaces in those nodules which are 

 similar in structure and composition to the surrounding rock. Professor 

 BONNEY'S observations on the nodular felsites of North Wales led him 

 to the conclusion that the nodular structure had been produced in 

 two ways : 



(1) By simple contraction and roughly concentric cracking of the 

 mass on cooling, being thus intermediate between the perlitic structure 

 common in glassy acid lavas, and the spheroidal structure common 

 in basalt. 



(2) By similar contraction in cooling, which is determined by the 

 presence of a cavity, and produced as follows : When the cavity is 



(1) H. LOKETZ. Jtihr. <1. k. preuss. Geol. Landesanstalt fur 1881, p. 11 h. Berlin, 1882. 



(2) A.J.S., 3rd Series, Vol. XXX! IT., 1S87, p. 3(1. 



