1882.] Ill [Zirkel. 



cepted in Europe and was not adopted by Zirkel until his visit to 

 New York " (b. 246J. Whence does Mr. Wadsworth derive his 

 authority for writing such a totally unjustified phrase? Iiicht- 

 hof en's theory has met with warm sympathy among German geolo- 

 gists; and as for myself, since the time (1867) when my eminent 

 friend sent me a copy of his admirable " Natural System of Vol- 

 canic Rocks," I have always considered the chief results of his 

 investigations as an extraordinary step in scientific progress. It 

 is really not easy to understand how Mr. Wadsworth can assume 

 the boldness to say that all my previous work had been done ac- 

 cording to a different method of classification (b. 249). That 

 arrangement of the feldspar-bearing rocks which I have given on 

 page 6 of the incriminated Vol. vi (of the Geol. Expl. Fortieth 

 Parallel) is literally the same as that which I have used in my 

 " Mikroscopische Beschaffenheit der Mineralien und Gesteine," 

 1873, page 290, and which since the year 1863 I have always made 

 the foundation of my University lectures on Petrography. 



4. Mr. Wadsworth gives to his readers an account of how the 

 work on the lithological collection of the Fortieth Parallel Sur- 

 vey was done (b. 248). This narrative, either by unacquaintance 

 with the facts or by accident, is incorrect in one rather important 

 point. In the summer of 1874, many of the sections prepared in 

 America were sent to me in Leipzig ; they bore only a number 

 with no lithological name, and they were not accompanied by any 

 catalogue. Of this collection of slides, which represented all the 

 different occurrences, I made a preliminary study before I went to 

 New York, endeavoring to determine the petrographical nature 

 of the thin sections without any knowledge of the hand-speci- 

 mens of the rocks from which they were derived, and without any 

 acquaintance whatever with their field relations. Afterwards, 

 through personal intercourse with Mr. King, it afforded me much 

 pleasure and gratification to learn that my diagnosis, in the ma- 

 jority of cases, had been made quite correctly : that those sec- 

 tions which I had determined in Leipzig as belonging to diabases 

 were, according to the results in the field, really pre-tertiary pla- 

 gioclase augite rocks ; that those considered by me as basalts 

 indeed belonged to the tertiary epoch, and that my microscopical 

 distinctions between the sections of felsite porphyries and those 



