1883.] 423 [Wadsworth. 



basalt." There is a horn to this dilemma which Dr. Zirkel does not 

 seem desirous to take. If my paper of 1879 be turned to, it will 

 be seen that basalt is essentially defined there as a rock which in 

 the unaltered state is composed of plagioclase, magnetite, olivine, 

 augite, a globulitic base and a little sanidin, while andesite is de- 

 scribed as composed of a glassy or felsitic (felty) base with vary- 

 ing augite and hornblende, j)lagioclase, more sanidin than in the 

 basalt, and magnetite. Trachyte is said to have a light gray 

 glassy base with predominating sanidin, biotite, and hornblende, 

 while rhyolite is stated to possess a clear glassy base holding 

 crystals and fragments of quartz, sanidin, biotite, and horn- 

 blende. All these are said to have their old and altered states, 

 since the key note of that paper is that the older rocks now 

 given distinct names are really rocks that once were identical 

 with modern forms — the present difference being due mainly, 

 if not entirely, to alteration and crystallization. Now after 

 giving these definitions it is stated that Zirkel has described 

 rhyolites as trachytes, ^trachytes as andesites, and rhyolites 

 and basalts as trachytes, etc. After the remark that most of 

 the trachytes analyzed belong to the basalts and andesites it 

 is said : " The basalts that have been described as trachytes are 

 not doubtful ones, that might naturally have been classed with 

 trachytes, but well-marked specimens, identical with some that 

 have been described under the head of basalt in the same report." 

 (B. 278-281, 285, 286.) Now is not my meaning clear that these 

 rocks are not those in which sanidin predominates, but that Zir- 

 kel incorrectly determined the feldspars, and took plagioclase for 

 orthoclase — a mistake which I found he had repeatedly made in 

 both the older and younger rocks. Again some of these rocks 

 contain olivine according to Zirkel's own statement (W. 267), 

 and, therefore, ought to be basalts according to him. 



I did not ask how the absence of olivine absolutely separates a 

 rock from the basalts as Zirkel says I did (Z. 115) ; but I did ask 

 how the absence of olivine in a rock absolutely separated it from 

 the basalts in Zirkel's own work, as he said it did, when he de- 

 scribed a number of rocks as basalts which contained no olivine 

 according to to his own testimony ! (W. 267.) His remarks now 

 show that in his opinion he must have made serious mistakes in 

 this work and in many of his former papers, since Rosenbusch 



