114 PLE i OOTINALE TOL GARLOIEO GV 
gneisses are mainly rocks of igneous origin. While recognizing the 
undoubted evidence of secondary dynamic action in many regions, 
and the absence at present of criteria by which original and secondary 
structures may be discriminated, the authors are strongly of the opinion 
that much of the banding of gneisses, as distinguished from mere 
foliation, may be an original structure due to the conditions in which 
the igneous magma was erupted and consolidated. 
The necessity of establishing the prevalence of such differentiated 
lamination in basic rocks, and of recognizing its occurrence to any con- 
siderable extent in granitic masses, is self-evident after the bearing of 
such facts on the nature and origin of the ancient gneisses has been 
so clearly set forth. 
JosEPpH P. IDDINGs. 
Preliminary Report on the Geology of South Dakota. By J. E.Tonp, 
State Geologist. (South Dakota Geol. Surv., Bul. No. 1, 
172 pp., 5 plates, Prelim. Geol. Map. Sioux Falls, 1895). 
Summaries of the progress up to date along a particular line of 
work, or of the knowledge of the geology of particular regions are 
always welcome. ‘This is particularly true when the report covers a 
region of so wide and varied interest as South Dakota and one the 
literature of which is so badly scattered. Since the early work of 
Hayden in the eastern and of Newton and Jenny in the western half 
of the state the papers on the geology of South Dakota which have 
appeared have been fragmentary only. ‘They have, however, modified 
our ideas of the geology of the region in many important regards. As 
a foundation for the future work of the Geological Survey Todd has 
brought together in convenient form all this mass of information and 
has added to it, asa result of his several years work in the region, 
a great deal that is now for the first time published. 
Among the new points which may be noticed are, the recognition 
of the Silurian as present in the Deadwood section and the pointing 
out of the beds which must represent the Devonian if it be at all 
present. A number of caves in the Carboniferous are described in 
detail. In one of them, Wind cave, is a curious calcite formation 
called “box-work.” The peculiar dome-like surface of the Purple 
Limestone, it is suggested, may be due to the leaching out of salt beds 
of irregular thickness below. The marine origin of the Dakota is 
