424 S. ALLPOKT ON THE METAMOEPHIC EOCKS 



phyritic texture, having conspicuous crystals of felspar scattered 

 through it. Both varieties are highly altered : the secondary pro- 

 ducts are serpentine and ealcite ; and in some specimens there is a 

 little quartz ; characteristic pseudomorphs after augite are abundant, 

 and, as not unfrequently happens, the triclinic felspar has suffered 

 less than either of the other constituents. 



Summary and Conclusion. 



Microscopical examination shows clearly that the pyroxenic mine- 

 ral, whether augite or diallage, has frequently been converted into 

 a hornblendic substance, and that the variety actinolite is found 

 filling cavities and fissures precisely in the same manner as other 

 products of alteration. 



That the more compact varieties of these metamorphosed dolerites 

 or basalts should exhibit an imperfect cleavage which is entirely 

 absent from the coarsely crystalline portions, is quite in accordance 

 with the facts observed in typical slates. In such rocks it is found 

 that even a slight change of texture in the beds is frequently accom- 

 panied by an alteration in the direction of the cleavage-planes ; and 

 it is no unusual circumstance to meet with coarse uncleaved gritty 

 bands lying between fine slates, and abruptly terminating a plane of 

 cleavage, which is as suddenly resumed on the other side of the 

 band. 



All the metamorphosed dolerites which occur in the altered Devo- 

 nian slates have been collected at a greater distance from the granite 

 than the schists previously described; the metamorphism is of a 

 different kind, and there is but little evidence of any direct action 

 on the part of the intruded rock. The alteration that has taken 

 place appears to be the result of internal rather than of external 

 action ; in other words, it must have been caused by a more or less 

 complete decomposition and rearrangement of mineral substance in 

 situ, and not to any great extent by the introduction of new material 

 from without. 



Some of the facts observed in the course of these investigations 

 are singularly interesting and important from a petroiogical point of 

 view. 



By an examination of a sufficient number of specimens the pro- 

 cess of alteration may be followed step by step from a slight external 

 change in a single mineral to the production of a perfect pseudo- 

 morph, and ultimately to the complete transformation of the entire 

 mass of constituents, the result being the formation of a metamor- 

 phic rock whose original composition and structure could only be 

 detected by a study of a good series of the intermediate forms. In 

 a previous communication to this Society*, I have described a number 

 of pseudomorphs after augite and olivine, and the frequent diffusion 

 throughout the surrounding mass of various chloritic, serpentinous, 

 or other green- coloured products of their decomposition ; but in the 

 more highly altered rocks of Cornwall, it becomes easy to trace new 



* Quart. Journ. Geol. Soc. vol. xxx. pp. 535-542. 



