234 Proceedings of the Boyal Physical Society. 



at the edges of the mass, and sends out several apophyses of 

 felsite, which are of a yellowish brown tint, and to the naked 

 eye almost undistinguishable from fine sandstone. 



Orthoclase porphyry — the so-called grey porphyry of the 

 Harz — occurs in a series of dykes crossing the Harz be- 

 tween Ilfeld and Wernigerode. It has for ingredients 

 orthoclase, oligoclase, quartz, a dark green hornblende-like 

 mineral, mica, and in lesser quantity, graphite, pinite, and 

 garnets. 



Basalt dyhes run parallel with these, and are probably con- 

 nected with the interbedded basalts of the Lower Permian 

 group at Ilfeld. 



The term hasalt seems to me to be preferable to the 

 old word melaphyre, which is in general use in Germany. 

 There is really no petrographical difference between the two 

 rocks, the term melaphyre being simply used as a synonym 

 for basalt of Palaeozoic age. To classify rocks on chronological 

 grounds seems to be a false principle. Sandstone is surely 

 sandstone, and limestone, limestone, whether it be Palaeozoic 

 or Pleistocene, and why should basalt not always be called 

 basalt too, so long as it is really the same petrographical 

 species ? Speaking from the point of view of a geologist 

 and not of a petrographer, I am of opinion that the name 

 "melaphyre" should either, like the terms "trap" and "green- 

 stone," be laid on the shelf or else be used in an altogether 

 different sense, as its present signification can only be 

 productive of uncertainty and ambiguity in geological 

 investigation.^ 



Porphyrite occurs at Ilfeld in large interbedded sheets 

 overlying the basalt of the Lower Permian series just de- 

 scribed. It has a brown, reddish grey, or greenish ground- 

 mass, with decomposed porphyritic crystals of plagioclase and 

 hornblende, and more rarely ironglance and garnet. The 

 microscope shows the ground-mass to consist of orthoclase, 

 plagioclase, a little hornblende, quartz, magnetite, apatite, 

 and occasionally graphite and titaniferous iron. 



^ Compare Zirkel, Mik. Beschaff, p. 411 ; Rosenbusch, Mik. Phys. der 

 massigen Gesteine, p. 392 ; A. Geikie, Text-book, p. 14(5. 



