544 S. Allport — On the Igneous Rocks of Arran. 



are quite opaque ; others are glassy in tlie middle, with an opaque 

 coating. The dark colour is due to the presence of a great number 

 of grains of magnetite and a little chloritic mineral scattered through 

 the base ; there are also a few crystals of a pyroxenic mineral altered 

 to the same green substance. The base exhibits a more crystallized 

 structure than the rock from Benan Head, and in this respect resem- 

 bles the felsite from Auchenhew Hill previously described. 



It should be distinctly understood that in calling these rocks 

 felsites or quartz porphyries, nothing is implied thereby as to their 

 age ; they might with equal propriety be called trachytes, and 

 would certainly be placed in the trachytic group, if they were 

 known to be of Tertiary age ; there is no mineral ogical or structural 

 difference between them and the recent trachytes, nor on the other 

 hand is there anything to distinguish them from much older rocks. 

 If any petrologist thinks he can determine the age of such rocks by 

 their chemical or mineralogical composition, he might try his skill 

 on these Arran felsites before their age is ascertained by other means. 

 So far as is yet known with certainty, they may have been intruded 

 at any time between the close of the Carboniferous and the close of 

 the Tertiar}?- Periods ; there is here, therefore, an excellent oppor- 

 tunity of solving a difficult problem, and any one who has discovered 

 a criterion for determining the age of eruptive rocks would render 

 great service to science by making it known. Although there is at 

 present great confusion in petrological nomenclature, a partial 

 alteration, or introduction of new terms, would only make matters 

 worse ; and as more exact methods of observation are now being em- 

 ployed, it will not be very long before the composition and structure 

 of most rocks will be known. Petrologistswill then be able, for the 

 first time, to adopt a nomenclature founded on a knowledge of facts ; 

 for it should be remembered that most of the names by which the fine- 

 grained rocks are known were given in complete ignorance of their 

 true composition. In the meanwhile no harm can be done by discon- 

 tinuing the use of a number of terms which have lost, or never 

 had, any definite meaning ; for example, the names ' porj)hyry ' and 

 ' porphyrite ' may be dismissed as quite unfit for generic terms, the 

 structure to which they are applied being frequently found in all 

 kinds of igneous rocks, and therefore characteristic of none. It 

 should be restricted to its proper use as a varietal character only, 

 and such names as 'felspar-porphyry,' 'pitchstone-porphyry,' etc., 

 should give way to 'porphyritic felsite,' ' porphyritic pitcbstone,' etc. 

 Another step would be to ignore the distinction now made between 

 rocks of different ages, when there is really no essential difference 

 between them ; we should then get rid of melaphyr, aplianite, 

 anamesite, diabase, and greenstone. The three former are but va-. 

 rieties of ' dolerite,' ' diabase ' is simply an altered ' dolerite,' while 

 ' oreenstone ' has now no definite meaning whatever, having been 

 applied to diorites, dolerites, felsites, or in fact to any rock which a 

 collector or writer could not make out. An indefinite term is of 

 course frequently useful in the field, and perhaps none could be 

 better for the purpose than trap. 



