6 GEOLOGICAL *MEMOIRS. 



The author then proceeds to describe the characteristic features of 

 these three subdivisions of Felsite-porphyry, commencing with the 

 quartziferous, to which indeed his remarks are principally confined. 



With regard to the mass or paste of quartziferous porphyry, he 

 remarks (p. 18), that it has been already observed by Heim, that 

 ** in porphyry everything is extremely changeable. In the same bed 

 the mass occurs sometimes in a coarse, sometimes in a fine-grained 

 condition, here argillaceous and soft, there hard and producing a 

 spark from steel*." Whoever has traversed a porphyry district must 

 have remarked how constantly every hill, or even every rock, offers 

 fresh varieties and new peculiarities ; that the same rock appears in 

 one place compact, hard, and close-grained, in another soft and earthy, 

 and again in another place porous ; that in one place numerous in- 

 gredients occur, which entirely disappear after a short distance. The 

 Bruchhaus Stones, where the rock appears under more different 

 aspects than are generally met with within so small a space, are an 

 excellent example. At the same time in many porphyry districts, 

 regular characteristic transitions from one to the other condition can 

 be established ; thus Credner reckons six principal varieties in the 

 Thuringian Hills, whilst Naumann distinguishes eight mineralogi- 

 cally different porphyries in the neighbourhood of Wurzen, Grimma, 

 and Taucha. 



The metamorphism, mineral contents, and varied structure of 

 porphyry are then briefly but clearly described, together with the 

 substances into which the felspar is occasionally changed. 



The changes which the felspathic substances undergo produce 

 various results, leading to the formation of different minerals, viz.: — 

 1. Kaolin; 2. Lithomarge ; 3. Steatite [Speckstein] ? ; 4. Alum. 

 These are as fully described, pp. 28-31, as the present state of mi- 

 neral analysis allows, although the origin of the talc in the speckstein 

 has not yet been sufficiently explained. 



The different conditions of the occurrence of quartz and mica in 

 porphyries are next described (p. 35-40). Quartz occurs more re- 

 gularly and systematically than either felspar or mica, and almost 

 universally in the form of bipyramidal dodecahedral crystals, seldom 

 transparent, and only rarely with the six-sided prisms. While quartz 

 and felspar may be considered as the most important contents of 

 quartziferous porphyry, mica can only be considered as a subordinate 

 ingredient, although as such it is one of the first. It generally occurs 

 in small scales, leaflets, or six-sided tablets. 



Amongst the less important minerals which occur in quartziferous 

 porphyries, either as nodules or in veins, the author mentions (p. 41- 

 45) hornblende, pinite, garnet, iron-pyrites, magnetic iron, graphite, 

 tourmaline, agate, carnelian, plasma, opal, fluor-spar, chalcedony, 

 barytes, oxides of iron, epidote, psilomelane, gypsum, calc-spar, pin- 

 guite, chrome-ochre, native copper, &c. 



The granitic and unquartziferous porphyries are next briefly de- 

 scribed, together with the varied structure and fracture of the differ- 

 ent porphyritic rocks (p. 45-63) ; after which the author points out 

 the various localities where porphyry has hitherto been observed on 

 * Beschreibung der Thiiringer Waldgebirges, vol. ii. pp. 44, 50. 



