Correspondence. 125 
and geological evidences of the consanguinity ot igneous rocks are fully 
discussed. This term is used to express the idea that all the igneous 
rocks of any volcanic district have been derived from a single stock- - 
i. e., from a common but gradually varying magma. The localities from 
which most of the illustrations are drawn are two regions with which 
the author is very familiar, — Electric peak and Sepulchre mountain, and 
Orandall basin. Following this is a consideration of the differen- 
tiation of molten magmas, the cause, processes and results of 
differentation and the localization of volcanic phenomena. Argu- 
ments are presented to show that the compounds of a magma do 
not exist as definite mineral molecules, nor even as fixed silicate 
molecules, but as simple oxide molecules, which arrange them- 
selves in different associations according to physical circumstances; 
and that differentiation does not go on in accordance with definite stoi- 
chiometric proportions. The conclusion is that the character of the 
differentiation of molten magmas is "that of a chemical separation 
affecting the elementary oxides of the constituent elements," and that 
the magma is a solution of these oxides. In this the author differs 
from the ideas of Bunsen and Lagorio and those usually accepted, — 
i. e., that a molten magma consists of a solution, or perhaps an alloy, of 
the different fixed mineral compounds, the solvent being an acid silicate 
of potash and soda. He also differs from the recently published views 
of Rosenbusch, one of the recognized leaders in the study of igneous 
rocks, and concludes that Rosenbusch's idea of the existence of certain 
definite, stoichiometrically proportioned molecules, termed "kerns," 
which are capable of dissolving any of the oxides or metals in rocks to 
varying extents, is without foundation. Differentiation in a molten 
magma must have gone on when the magma was still fluid and is not 
due to the crystallization of certain minerals and their subsequent ac- 
cumulation by gravity or other means in certain areas, as has been 
suggested by Brogger and Vogt. The cause of differentiation is thought 
to be differences in physical condition in different parts of the magma, 
and the means, as first suggested by Lagorio, is the application of the 
principle of Soret, who found that if two parts of a solution of any salt be 
kept at different temperatures there will be a concentration of the salt 
in the cooler part of the solution. 
In the views as to the consanguinity of the igneous rocks of any area 
and the idea that a molten magma is not composed of solutions of fixed 
mineral compounds, Mr. Iddings has taken a step in advance of the 
thinkers in this realm, and too much cannot be said in praise of his ex- 
cellent article. A valuable part of the paper is the selected tables of 
accurate rock analysis and also the diagrams of molecular variations in 
certain groups of igneous rocks. 
CORRESPONDENCE. 
The Formation of a Terrace.— On the southeast shore of a lake 
lying in the S. E. J4 of Sec. 3 in the township of Aastad, Otter Tail 
county, Minnesota, is a remarkably well developed beach ridge for a lake 
