8 GEOLOGICAL MEMOIRS. 



1. Quartzite and porphyry ; 2. Diorite ; 3. Melaphyre ; 4. Basalt; 

 and 5. Pitch-stone, with porphyry. 



IV. Mutual relations of different porphyries one to another. 



1 . Micaceous porphyry and quartziferous porphyry. This occurs in 

 Thuringia, where the latter is almost always the youngest rock. 



2. Quartziferous porphyries of different ages. 



The concluding portion of the work relates to the occurrence of 

 ores in porphyry, and the relation of the latter to metallic veins. 

 Contrary to the opinion of former writers and authorities, recent in- 

 vestigations have shown that the quartziferous porphyries not only 

 frequently contain metallic veins, but that, in many districts, they 

 must be looked upon as the true agent in the production of ores. In 

 Siebenbdrgen (p. 187) the auriferous rock is a soft, white, argillaceous 

 porphyry. The mountain Affinish, which rises 300 feet above V6- 

 rospatak, consists of clay-porphyry, with many large crystals of 

 quartz. The rock is perforated by numerous narrow crevices, which 

 are often filled with carbonate of manganese, and also contain native 

 gold and lead ore*. In Mexico the quartziferous porphyry near Qua- 

 naxuato is intersected by veins of calcareous spar and quartz, which 

 also contain gold and silver. 



We cannot conclude these extracts from Dr. Leonhard's work 

 without one more quotation respecting the localities from whence the 

 porphyries of the ancients were derived. Referring to the use of 

 porphyry by the Romans, our author says (p. 207) : — It is less gene- 

 rally known that the Romans also obtained porphyry from France, 

 which was used in their works of art. In this respect the commu- 

 nications of Coquand and Texier are of much interest. Between 

 the base of the Esterel group, consisting of red porphyry, and the 

 sea, there occurs a bluish-green porphyry. It is generally called 

 Porphyre des Caux. This fine stone, susceptible of the highest de- 

 gree of polish, was used by the Romans to adorn the harbour of 

 Frejus, and, amongst the many valuable materials used to embellish 

 and enrich the town, there is none to be compared with this bluish- 

 green porphyry, which was used for numerous monuments (according 

 to Coquand, also to ornament the monuments at Riez, Aix, Aries, and 

 Orange). The stone was even transported to Rome, where, as was 

 even the case in Frejus, it was considered to have been obtained from 

 Egypt (amongst other instances, Coquand mentions several columns 

 in St. Peter's and in the Quirinal as made from the porphyry of 

 Frejus) . It was for a long time unknown that the quarries were in 

 the immediate neighbourhood of the town. Strangers took away 

 specimens in the conviction that they belonged to a rock the locality 

 of which was unknown. The old quarries are, however, still extant. 

 They are situated in a deep valley, distant 10 70 metres from the sea- 

 shore. Detached blocks were here found, as well as iron wedges, 

 driven into (and still remaining in) the cracks and fissures of the rock 

 to facilitate the removal of large masses. An ancient vase, fragments 

 of red earthenware, and a bronze medal bearing the head of Vespa- 

 sian, found in the quarries, prove that they were worked, or had at 

 least been opened, in the time of the Csesars. 



[W. J. H.] 



* Lill V. Lilienbach. Boue, Mem. de la Soc. Geol. de France, vol. i. p. 276. 



