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L. V. Pirsson—Phonolitic Rocks from Montana. 399 
The ground-mass in which the above phenocrysts lie appears 
much like that of the preceding rock, it is a compact felt of 
small laths of orthoclase (probably with more or less anortho- 
clase) and slender needles of sgirite often arranged in beauti- 
ful flow structures. Scattered through this are small irregular 
patches of quartz. This rock, on treatment with acid, does 
not show a trace of gelatinization, thus confirming the absence 
of nephelite. The quartz, of course, was optically proved. 
Use of the term “ tenguaite.”—Asis well known, Rosenbusch* 
first gave this term to phonolitic rocks from Brazil previously 
described by Graefi.t According to the idea expressed by 
ote they filled a definite position in his system of “dike” 
rocks. 
Later, however, Derby,{ from whom the original types had 
been received, states that they occur in effusive masses as well 
as in dike forms. On this account some writers have rejected 
the term. It seems tous, however, that as a name by which to 
designate those fine-grained porphyritic rocks which contain 
large amounts of eegirite in the ground mass, if used without 
regard to their geological mode of occurrence, the name is a 
very useful one for field and petrographic purposes and deserves 
to be retained. As a sequence of the abundance of egirite 
needles in the ground-mass, such rocks have a distinct megas- 
copic habit and color that it is very useful to recognize.§ 
* Mass. Gest., p. 628, 1887. 
+ Jahrb. fiir Min., 1887, vol. ii, p. 257. 
¢ Quart. Jour. Geol. Soc., vol. xlvii, p. 254, 1891. 
§ While the above article was passing through the press the author received 
Prof. W. C. Brégger’s important and interesting work ‘‘ Die Gesteine der Grorudit- 
Tinguait Serie.” a previous sending having been lost in the ill-fated Elbe. The 
quartz tinguaite described above corresponds very closely with the ‘ Grorudite”’ 
of Brogger, differing chiefly in the presence of the large feldspar phenocrysts. 
Brogger recognizes the difficulty of differentiating the phonolites occurring in 
flows from those in dikes and proposes to divide the group into those rich in lime 
(carrying hauyn and nosean) and those poor in lime (0°5-2°0 per cent) but rich in 
zegirite and nephelite, and thereby having the characteristic green color. The 
term tinguaite is to be reserved for the latter. This is essentially the same idea 
as that proposed above though in a somewhat more limited sense. 
In adverting to Derby’s article Brégger states that Derby does not say that the 
Brazilian tinguaites occur in effusive flows. This is true, but it is nevertheless 
precisely the idea that the writer, along with others, understands that he wishes 
to convey. It is to be hoped that Prof. Derby himself will see fit to clear up 
this matter, about which so much misapprehension seems to exist. 
Mineralogical-Petrographical Laboratory, 
Sheffield Scientific School of Yale University, New Haven, July, 1895. 
