MUSEUM OF COMPARATIVE ZOOLOGY. 285 



paragouite schist from St. Gotthard. It is, furthermore, destitute of 

 distheue, although he says it coutaiiis " excellent large crystals of pale- 

 blue disthene." 



His granite porphyries belong in part to granites and gneisses and in 

 part to the felsites, and his syenites are old and much decomposed 

 andesites, in which the quartz is an alteration product. 



The diorites are partly sedimentary rocks, partly granites and felsites, 

 partly old andesites, and the rest are old basalts. 



The so-called hornblende porphyries are somewhat altered andesites, 

 and the diabases are mainly altered basalts, but a few are unaltered 

 ones, the remainder being altered old andesites. The melaphyrs are all 

 old, altered basalts, except one, which is an old andesite. 



The propylites are all altered andesites, with which species their 

 chemical composition agrees ; and the diagnostic distinctions that Pro- 

 fessor Zirkel has placed between the andesites and propylites do not 

 hold good even in the specimens that he described, as would have 

 been readily seen had he given complete descriptions instead of the 

 very imperfect and often inaccurate ones that have been published. 

 The distinction between these rocks is simply in the degree of altera- 

 tion, and they pass directly into each other. With but two exceptions, 

 the quartz propylites are old granitoid, felsitic, and fragmental rocks, 

 while the two exceptional ones are altered andesites, in which the quartz 

 is an alteration product. Rejecting, then, the so-called propylites 

 that are not propylites, the range in silica is from 58.66 per cent to 

 64.62 per cent, or about 6 per cent, as shown by Mr. King's list of 

 analyses. 



No line can be drawn between the hornblende and augite andesites, 

 but both form one continuous series. Professor Zirkel being mistaken in 

 his statement that the augite andesites are younger than the rhyolites. 

 Beginning with the andesites, the volcanic rocks have been described 

 and arranged very indiscriminately, rhyolites being described in his 

 report as trachytes, trachytes as andesites, and rhyolites and basalts as 

 trachytes, etc. Only a few of these cases can be pointed out here. All 

 the dacites, with one exception, are rhyolites, felsites, and fragmental 

 rocks not belonging to andesites. The exception is an altered andesite, 

 in which the quartz is an alteration product. Referring again to Mr. 

 King's tables of analyses, we find the range of silica in the undoubted 

 andesites to be from 58.33 per cent to 62.71 per cent, or a little over 

 4 per cent. Analysis 140 is that of a rock referred with doubt to the 

 andesites. 



