EDITORIAL COMMENT. 
ARCHEAN ERUPTIVE ROCKS OF FINNLAND. 
Studien ueber achaische Hruptivgesteine aus dem sud westlichen Finnland. 
yon J.J. SEDERHOLM. (Minerulog. und petrogr. Mitth. XIT, 1891. 
This paper is an interesting and clear statement of the results 
of detailed geological and petrographical.studies, in a field whose 
general features have already been described and mapped by Dr. 
Sederholm, in the Swedish language. 
The Archean age of the rocks investigated is determined by 
the geological condition which obtains generally throughout very 
extensive regions in Finnland, viz: that the Cambrian and Silurian 
rocks are found constantly in a perfectly horizontal and undis- 
turbed attitude, while the underlying pre-Cambrian have been 
much folded and altered. In the field described by Dr. Seder- 
holm the immediate superposition of the Cambrian is not ob- 
served, but the rocks he treats of are identical petrographically 
and in the conditions of their occurrence, disturbance and altera- 
tion with rocks the pre-Cambrian age of which is demon- 
strated; so that he infers them to have been in existence prior 
to the great epochs of disturbance and erosion, which antedated 
the Cambrian in this part of Kurope, and hence Archeean. 
Among the different formations which the author recognizes 
and describes may be mentioned first a series of phyllites, mica 
schists and hornblende schists, all of which appear to be altered 
sedimentary rocks. These altered sedimentary formations are 
surrounded by granite rocks, which penetrate them in innumer- 
able dykes and veins, both transverse and parallel to the planes of 
schistosity. Fora-portion of these granites the term ‘‘Adergueiss”’ 
is used as descriptive of its intimate interveining with the schists. 
Occurring in extensive masses there are two chief varieties of 
granites of quite different age, viz: (1). A gray granite, rich in 
plagioclase and having generally hornblende as a constituent. 
This possesses often such a well marked parallel structure that it is 
commonly alluded to as gneiss. This parallel structure is held to 
be a pressure effect. In the midst of this granite there occur 
masses of basic rocks sometimes several kilometers in extent, but 
generally much smaller. These pass by gradations into the envel- 
oping granite, and are held to be more basic separations or seere- 
