346 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



ian periods, and even earlier, although in lesser quantity among 

 the primitive slates. In the Carboniferous system, they are in- 

 truded almost as frequently as the porphyries ; but towards the 

 commencement of the Permian period, they seem to be repl aced 

 by melaphyres, which continue to be erupted even as late as the 

 Triassic period. The circumstances attending the protrusion of 

 the greenstones and melaphyres are essentially the same as in the 

 case of the porphyries. The strata which the former have broken 

 through furnish abundant evidence of the extraordinary force which 

 ejected them, and the dislocated strata also occasionally furnish 

 proofs that they have been chemically acted on by the plutonicrock. 

 This latter is especially the case with the melaphyres, which have fre- 

 quently carbonized the coal and hardened the clay slates with 

 which they have come in contact, in the same manner as more 

 recent eruptive rocks. In the following tables will be found the 

 names and characters of the rocks referred to in this paragraph. 

 Massive rocks of the Porphyry class* 



NAME. CRYSTALS OCCURRING CHARACTER OF THE PASTE. 



IN THE PASTE. 



Quartz porphyry. Quartz and Feldspar. Yellow, brown and red 



colored. 

 Syenitic porphyry. Quartz, Chlorite f Brown or green ; 



Feldspar, sometimes -{ somewhat granular. 

 Mica. { 



Granitic porphyry. Quartz, Mica,Feldspar. Sometimes granular. 



Micaceous porphyry. Mica and Felspar. Brown coloured. 



Minette. Mica and Felsite. 



Hornblendic porphyry. Hornblende & Feldspar. Dark coloured. 

 Felspathic porphyry. Feldspar. 

 Felsite rock. Sometimes Quartz. Yellowish, reddish, 



or greenish-grey. 

 Pitchstone,and Pitch- ^ Glassy Feldspar, Quartz 

 stone porphyry. £ and balls of Felsite. 



Rochs made up of Porphyritic debris.\ 



Porphyry breccia. 

 Porphyry conglomerate. 

 Porphyry sandstone (psammite.) 

 Porphyritic tuff or felsite tuff. Claystone. 



Massive rochs of the Greenstone and Melaphyre class.\ 



NAME. ESSENTIAL CONSTITUENTS. TEXTURE. 



Diabase. Augite, Labradorite, and Granular, porphyritic 



Oligoclase. and slaty. 



* Cotta, Gesteinslehre, p. 97. 

 t Naumann, Lehrbuch, I, 706. 

 % Cotta, Gesteinslehre, p. 47. 



