348 A. K. Wells— 



Cambrian strata of the Fichtelgebirge. Later, Lessen ^ showed that 

 similar rocks occur in the Middle Devonian strata of the Harz. 

 In commenting upon the chemical analyses of these, the original 

 keratophyres, Teall ^ emphasizes the wide range of silica percentage, 

 varying from 50 to 70%, and the large proportion of alkali felspar, 

 calculation showing the presence of from 74 to 92%. Considerable 

 variation in the soda : potash ratio is another noteworthy feature 

 of these rocks, sometimes the former, sometimes the latter being 

 in excess. It is evident, therefore, that the name keratophyre was 

 given to an ill-defined group of rocks varying widely in chemical 

 and mineral composition as well as in mode of occurrence. 



It is interesting to note that Teall, after stating that keratophyres 

 had not then (in 1888) been discovered in this country, suggested 

 their probable occurrence in the West of England. This prediction 

 has proved true, as keratophyric rocks have been found to be 

 abundant in the Newton Abbot district and to " characterize every 

 horizon at which volcanic activity is evident 'V although they are 

 rare in other parts of Devon and Cornwall. 



The first discovery of keratophyri(j rocks was recorded by Hatch, 

 who found them among the Ordovician soda-felsites (lavas), of 

 Wicklow.* His paper contains the statement that Rosenbusch 

 had foretold their discovery among the Welsh rocks. Within recent 

 years this prediction also has proved true. Thomas on Skomer 

 Island, Cox at Abereiddy Bay, and Part in the Prescelly Mountains 

 have described keratophyres from these several different parts of 

 South Wales, while Cox and the writer have claimed as keratophyres 

 lavas occurring on two horizons in Merioneth. 



In classifying the Skomer Island Volcanic Series, Thomas divides 

 certain of the intermediate sodic lavas into two groups, (a) soda 

 trachytes, and (6) keratophyres. It is stated that a family connexion 

 exists between the two,^ which grade one into the other. What, 

 then, is the basis on which the division is made ? The following 

 points of difference are emphasized : the less perfect development of 

 flow structure, the greater amount of chlorite and granular sphene, 

 and the better developed vesicular structure iia keratophyres. 

 These differences are comparative only, and it is doubtful if any of 

 them is of sufficient importance to justify the use of two names for 

 rocks which seem so much alike. That one name would serve for 

 both is evidently the opinion of some, as the suggestion has been 

 made that both should be termed keratophyres, following the 

 Continental usage of the word. Here, then, are two conflicting 

 opinions ; the first that " keratophyre " is far too comprehensive 



^ Lessen, Jahrb. k. preuss. Geol. Landesansl. fur 1884 (1885), p. 21. 

 2 Teall, British Petrography, 1888, p. 369. 



^ Flett, " Geo], of the Country around Newton Abbot " : Mem. Geol. 

 Surv., 1913, p. 57. 



* Hatch, Geol. Mag., 1889, p. 70. 



^ Thomas, Quart. Journ. Geol. Sac, vol. Ixvii, 1911, p. 193. 



