56 Forbes — On alleged Hydrothermal 



Mag.) may be, is difficult for a petrologist to understand, especially 

 since he is immediately informed that it " is simply an admixture of 

 hornblende with white and pink felspar," and further told that it is 

 "of a more granitoid and less basic character than the usually finer 

 grained and often dull earthy rocks of dykes, which here and there 

 intersect the coal-measures ;" and, to complicate this confusion, in a 

 foot-note adds, " most of the trap dykes of the Scottish carboniferous 

 strata, however, are not hornblendic, but augitic greenstones or 

 dolerites." 



In investigations where exactitude is essential, trap is an ex- 

 tremely vague name to designate rocks by, as was long ago admitted, 

 for as early as 1827, Brongniart ^ says, " Cest encore un tres mauvais 

 nom, etc," and the term " felspathic traps " for one of their many 

 varieties of same is certainly not an improTcment in the name of a 

 rock which, under all circumstances, is essentially felspathic, having 

 a felspathic base. The name trap, originally adopted from the 

 Swedish geologists, was understood to denote any compact dark- 

 coloured rock composed of felspar with augite. 



The name wacke is another, which has but little true signification, 

 further than being usually applied to any dirty, indistinct mass, in 

 which the structure is obliterated; it is evidently, however, a 

 favourite with the author, who writes of "decomposing wackes," 

 " calcareous wackes," " altered wackes," and " highly alkaline 

 wackes," etc. 



Again the term porphyry, when applied to rocks, is frequently 

 improperly employed ; when used as an adjective, to imply a definite 

 structure, it is understandable, as porphyritic greenstone ; but the 

 terms felstone porphyry, felspar porphyry or felspathic porphyry, 

 sound rather tautological. If the application of this term in petro- 

 logy, when used to designate a definite rock, is inquired into, it will 

 be found that it was originally used to denote such rocks as con- 

 sisted, like the true antique porphyry, of crystals of felspar set in a 

 paste of semicrystaUine or amorphous felspar. It is therefore diffi- 

 cult to imagine a rock more felspathic than one composed mainly of 

 felspar, and one might just as well write of quartzitic quartzites as 

 " of felspathic porphyries ; " again, what is to be understood by 

 the term " porphyritic felstones " as different from porphyries ? 



4. Geology. — It would be impossible to enter into an examination 

 of the geology of these memoirs without some local knowledge, 

 which the writer does not possess ; but before concluding he would 

 still enter his protest against some of the geological views put forth 

 by the author. 



Thus, at page 517 and following of the memoirs read before the 

 Geological Society, great stress is laid upon the circumstance that, as, 

 instead of being flattened and drawn out, the vesicles found occur- 

 ring in these rocks are spherical, and are so over considerable areas, 

 the rocks therefore cannot be trappean or igneous ; now surely, 

 any geologist acquainted with volcanic rocks knows that spherical is 



1 Classification des Roches, p. 63. 



