200 Prof. T. G. Bonney — On Dana's Classification of Rocks. 



parison with the other. For instance, I have examined rhyolites, 

 felsites, basalts and serpentines of very different geological ages, and 

 have found it impossible to draw any important lines of distinction 

 between them. 



Still, while granting this, there may be advantages in retaining 

 such a term as diabase, though we admit the rock's former identity 

 with dolerite, and cannot draw a hard and fast line between the two. 

 The latter difficulty does not meet us here only in Lithology, and it 

 is convenient to have a term to denote varieties which have under- 

 gone marked alteration. Some geologists, I have observed, seem to 

 forget that igneous rocks, as well as sedimentary, have their meta- 

 morphic representatives, and as a rule text-books do not sufficiently 

 call the student's attention to this point. 



With reference to Professor Dana's remarks on trachyte and 

 felsite, it seems to me that, while admitting the' chemical identity 

 and common origin of both, we may conveniently take the presence 

 of a glassy base as characteristic of the one, and of a micro- or 

 crypto-crystalline ground-mass of the other. It is true that the latter 

 may have resulted from subsequent devitrification, and a rock once 

 glassy may have become cryptocrystalline. In that case, however, 

 where we are convinced of the fact, we may either call the rock a 

 devitrified rhyolite, and place it among the metamorphosed igneous 

 division, or coin a single name to express the fact of change. 



Professor Dana criticizes the use of the term ' gabbro.' It is quite 

 true that there has been some uncertainty in this, and that the Italian 

 petrol ogists call 'granitone' the rock to which the Germans apply the 

 name gabbro. It is also true that gabbro is closely allied to dolerite, 

 and that diallage is only a variety of augite ; still, as this gabbro is a 

 rock of a very definite appearance in the field, and seems to be of 

 rather a different habit from dolerite, 1 which shades off through 

 anamesite into the finer crystalline basalts (I think a revision of 

 terminology needed here), the name appears to be useful. As regards 

 euphotide and the so-called saussurite, I cannot altogether agree with 

 Prof. Dana. I have examined a good deal of this rock in the field 

 and microscopically, and have no doubt the mineral is only an altera- 

 tion product from labradorite or anorthite, and the rock simply an 

 altered gabbro. In the field (and under the microscope) the felspar 

 may be seen gradually altering into saussurite, and the pyroxenic 

 constituent into some form of hornblende. It seems, then, to me 

 that gabbro may conveniently be retained for the name of a rock 

 closely related to dolerite, and euphotide applied to the metamor- 

 phosed variety. 



Professor Dana is very severe on geologists (especially micro- 

 scopists) for their use of the term plagioclase. I venture to think 

 we have a defence. Doubtless plagioclase is only a synonym for 

 triclinic felspar, but I think that its correspondence in form with 

 orthoclase renders it a more convenient term. We do not regard a 



1 I have never seen gabbro except under circumstances which suggested deep-seated 

 intrusion ; it seems to be the analogue of granite. 



