4 GEOLOGICAL MEMOIRS. 



thered, or altered porphyries. " The quartz in these porphyries," 

 he remarks in his observations on the Thuringer Wald*, "in the 

 form of dodecahedral crystals, is a good guide to enable us to recog- 

 nize the red porphyry, even where it no longer shows traces of its 

 former state, and where, without the quartz, one might be tempted 

 to confound it with the unquartziferous, black porphyry .'' For this 

 purpose, L. v. Buch adopted the expression Quarz-fuhrender For- 

 phyr (Quartziferous Porphyry) — undoubtedly one of the best and 

 most characteristic of the many names which have been given to this 

 rock. Its importance in a geological point of view was recognized 

 by most geologists ; thus we have Porphyre quartzifire, Forjido 

 quarzifero, &c. It was not, however, always adopted. 



In England the word " Porphyry" was very freely used ; rocks of 

 different kinds (diorite, &c.) were thus called ; so that in many de- 

 scriptions it is impossible to detect the nature of the rock. 



Certain quartziferous porphyries which occur in the mining districts 

 of Cornwall as veins, partly in granite, partly in clay-slate, have been 

 long there known under the name of " Elvans." We have in vain 

 sought for the origin of this term in English writers. Henwood ex- 

 pressly saysf that the etymology of the word is unknown. May it 

 not perhaps be derived from a place called " Elvan" ? Reuss says, 

 in his * Lehrbuch der Geognosie ;|;,' that porphyry occurs near Elvan, 

 in Westmoreland. 



But comparatively little importance was attached to the presence 

 of quartz. Phillips, in his * Outlines of Mineralogy and Geology §,' 

 has made some attempts to distinguish the different kinds of Por- 

 phyry ; and MacCuUoch justly complains of the too great extension 

 of the word " Porphyry || ." 



Notwithstanding the proposals of Gerhard, d'Aubuisson, and others, 

 respecting the varying and incorrect ideas of hornstone- and clay- 

 stone-porphyry, and notwithstanding the observation of L. v. Buch, 

 that the matrix or paste of porphyries was not uniform, these ex- 

 pressions were maintained, because geologists wanted proper terms 

 to distinguish Porphyry, according to its characters — crystalline and 

 compact (Felspar), hard and brittle (Hornstone), or earthy and de- 

 composed (Clay-stone and Clay-porphyry). We find these names 

 both in the various elementary works on geognosy, and in the many 

 valuable communications which we possess respecting the occurrence 

 of this rock. 



After detailing the various nomenclatures proposed and adopted by 

 different mineralogical and geological writers, the author proceeds, 

 p. 8 : — The want or rather the great poverty of quartz at once distin- 

 guishes such porphyries from the characteristic quartziferous por- 

 phyries, without either separating them from the group of Felsite- 

 porphyries, or, still less, uniting them with the black or Augite-por- 



* Taschenbuch fiir Mineralogie, vol. xviii. p. 455, 1824. 

 t Trans, of the R. Geol. Soc. of Cornwiill, vol. v. p. 27, 1843. 

 X 1805, p. 292. § Page 167, ed. 1826. 



II MacCulloch, On the Geology of various parts of Scotland ; Trans, of the Geol. 

 Soc. Lond. vol. ii. p. 415. 



