Correspondence. 59 
table or tables. Authors of works on determinative botany have set us 
the example of this sort of aid at such a stage of study. 
It is at once apparent that a proper compilation of aids for Selbststudi- 
rende furnishes occasion for models such as have never been constructed, 
to illustrate ellipsoids of elasticity, bisectrices, and other doctrines. It is 
even more apparent, that a demand exists for purchasable thin sections of 
minerals cut in definite directions. These might be accompanied by full 
printed explanations of phenomena presented in light of various kinds, in 
rays parallel or convergent, and in different positions of the nicols. These 
however, might be omitted if suitable book-helps were to be had. An 
undertaking of this kind is commended to those who have means and time 
for makiagthin sections. The writer does not ignore the value of Julien's 
numerous sections ; but we find them, with their accompaning descriptions, 
rather intended to illustrate to experts the nature of different rocks and 
minerals, than to explain to the learner how and why the appearances 
presented justify the conclusions announced. 
To the writer it seems that the petrographer who will give the public 
an elementary book conceived along the lines above indicated, will do 
American students a very great service. But American students are in no 
need of another work revealing an ambition to construct a fabric of mere 
solid logic, and a fear that due simplicity will be mistaken for limited 
knowledge. 
A work like Rosenbusch's Mineralien, presenting a thorough and com- 
plete digest of the doctrines of optical investigation possesses inestimable 
value. But it is rather a "manual" or "handbook" for consultation by 
investigators than an elementary guide for learners. It is not a book based 
on true pedagogic philosophy. Its general plan and frame-work are those 
of a full systematic treatise, without due cognizance of the historic laws 
of the acquisition of knowledge. The guiding principle of its method is 
logic rather than psychology. To the subject-matter the j ustice done is 
ample; to the student, scant. Similar statements may be made respecting 
E. S. Dana's able treatment of the optical properties of minerals. Mr. 
Hawes' admirable exposition, besides lacking the true method, is too brief 
for the necessities of the learner. We yet need a learner's guide to 
microscopic petrography. A, Winciieli-. 
Ann Arbor, Dec. 27, 1888. 
Further notes on *'a green quartzite from Nebraska.''' — I trust that a few 
additional notes, which have been suggested by Dr. Hicks's remarks on 
the "green quartzite," in the November number of the Geologist will not 
be out of place. 
Mr. Bailey's observations accord with my own, as regards the limit of 
the drift. Seethe map on page 393, Proceedings of A. A. A. S., vol. 
XXXIII [1884J. In the same connection it is interesting to notice that Gen. 
G. K. Warren who had traversed the region a little further north stated 
at the Chicago meeting of the A. A. A. S. [1868] that the Missouri marked 
the limit of the drift in that latitude, and accounted for it in a way quite 
similar to that independently arrived at by myself in the paper before 
mentioned; viz.: That the course of the Missouri was determined Ijy the 
