48 THE CANADIAN NATURALIST. [Vol. ix. 



diabase into granular diabase and diabase-porphyry or diabase- 

 porphyrite; felsyte into felsyte and felsyte- porphyry ; and so on, 

 as if the porphyritic structure were deserving of first prominence 

 in the question of division into varieties, even greater than mineral 

 constitution ; and sometimes it is even made the basis of a dis- 

 tinct kind of rock. But, first, this porphyritic feature is only 

 one grade in the crystalline condition, and is of no more value as 

 resrards rock-distinctions than other s-rades. 



Secondly, it is of far less importance in this respect than any 

 variations in chemical or mineral compositions, such as are made 

 the basis of other varieties. 



Thirdly, it has often little stability in a rock-formation ; for 

 transitions in a dioryte from porphyritic dioryte to non porphy- 

 ritic are often found to take place at short intervals, laterally as 

 well as vertically ; and so it is with other porphyritic rocks. — 

 Within three miles west of New Haven, Connecticut, a labradorite- 

 dioryte undergoes many such transitions in intervals of a few 

 rods, illustrating the little value of the distinction based merely 

 on this condition in the feldspar. Half a dozen miles farther 

 west there is porphyritic granite which graduates, in a few yards 

 at some points, into porphyritic gneiss (the crystals of orthoclase, 

 two inches long and three-fourths of an inch broad) and this last 

 graduates near by into ordinary gneiss ; and gradations from por- 

 phyritic to ordinary gneiss are very common in the region. Such 

 facts make it evident that the porphyritic structure is a charac- 

 teristic of little relative importance ; that a porphyritic variety 

 may have rightly a place on a level with other ordinary varieties, 

 but never above one based on variations in composition. The 

 porphyritic structure is an easy character to observe ; but this is 

 not an argument in its favor that science can entertain. Such 

 names as fehite-porphyre, amygdcdoporpliyre, granitoporphyre, 

 melaporphyre (this last signifying "black porphyry ") and others 

 (abbreviated sometimes to felsophyre, amygdalophyre,granopliyre, 

 etc.) have high authority. But they seem to belong rather to 

 books on polished stones than to scientific works on lithology. 



The occurrence also of the augite of an eruptive rock in dis- 

 tinct crystals, or of quartz in double pyramids, and other similar 

 cases, can have nothing more than a small varietal value. The 

 criterion — crystals or not — is sufficient to distinguish only varie- 

 ties in mineralogy : and lithology can rightly make no more of it. 



(^To he continued.^ 



