288 Geology of 



if they had "been flowing in stream -like beds from fissures in the older 

 sedimentary or basic rocks. They are accompanied by pitchstones, 

 which mark the lines by which they were erupted, or where they came 

 in contact with other rocks, and thus it is evident that they were 

 subject to the same rules as volcanic rocks of younger origin. 



Most of the higher ranges such as Eocky Peak, High Peak, Mount 

 Misery, consist of such rocks, which having, doubtless, flowed from 

 several centres, all forming rounded or conical hills, have cooled in that 

 form. Many of these rocks have a trachytic appearance, without doubt 

 from partial decomposition, to which they have been subjected for num- 

 berless ages, and which has affected them to a great depth. However, 

 in some favourable localities, namely, in deep gorges and landslips, I 

 was enabled to observe how this earthy appearance of the base gradually 

 changes into a true felsitic structure. This gradation can be more 

 easily seen in the different dykes of the same rock, which radiate from 

 the small outlier east of Eocky Peak, and which have filled up fissures in 

 the palaeozoic rocks. The selvage of these dykes assumes sometimes the 

 structure of an imperfect pitch stone, but the sedimentary beds close 

 to them have been very little affected, the only difference being that 

 generally they break into smaller polyhedral fragments than the same 

 rocks a few yards further distant. 



The principal centre of eruption appears to have been at the head 

 of Eockwood Creek, at the base of High Peak, which is here sur- 

 rounded by vertical cliffs very difficult of access. Here a synenitic 

 granite porphyry occurs, which, however, I was not able to find 

 in situ ; it is identical with the Forphyre Granitoide of French 

 authors. Dufrenoy's description of this rock from Signon and Aix 

 applies fully to our rock specimen, !N"o. 348. It has been traced 

 by him. and Elie^de Beaumont in its gradual change into porphyry 

 similar to that of High Peak, but never into granite. (See Flie 

 de Beaumont and Dufrenoy explication de la carte geologique de France, 

 I. 30). Similar rocks occur in the Thuringian Eorest, also together 

 with " Felsitporpliyr,'' (see Lehrlucli der Petrograpliie Zirkel, I. 528), 

 they contain quartz, felspar, and hornblende, the latter found some- 

 times in well-defined crystals, but mostly in scaly aggregations ; 

 the felspar is white, and, without doubt, Oligoclase. Besides 

 this beautiful rock there appear in these precipices some brecciated 

 porphyries, and others, more or less laminated ; also, in the so-called 

 Eocky Eidge east of High Peak some of the porphyries have a 



