PUBLICATIONS. 22, 
arkoses which have been converted into mica-schists. The change of 
feldspars into biotite and quartz is notable. 
The eruptive rocks cutting the Penokee series in the form of dikes 
and sheets are normal diabases, occasionally grading ifto gabbro. The 
chief feature of interest developed in their study is the fact that their 
freshness and alteration is closely dependent on the permeability of 
the adjacent rocks to percolating waters, showing that environment 
may be a more important element than age in the preservation of a 
rock. 
The formations composing the Eastern area of the Penokee series 
have been modified by contemporaneous volcanic action, and the accu- 
mulation of surficial lavas, both massive and fragmental. ‘These were 
of a basic character, some being porphyrites, others diabases, grading 
into gabbros that are considered to be deep-seated parts of the lavas. 
These rocks have been altered into greenstones. The other rocks of 
the Eastern area resemble those ot the Western area in petrographical 
characters. 
In the closing chapter of the monograph the flexures and faults are 
discussed, and the structure of the region is described. ‘The Penokee 
series is correlated with the Animikie series, and also with the Mar- 
quette. Other correlations are suggested. 
JosepH P. IDDINGs. 
Summary of Current pre-Cambrian North American Literature.* 
Cross? describes intrusive sandstone dikes in the Pike’s Peak granite. 
The material has all the characteristics of dikes. The larger number are 
a few inches or a few feet thick, but they vary from a film to those 
three hundred yards wide. Some of the larger have been followed for 
nearly a mile: The dykes have a general trend parallel to the belt in 
which they occur, and they are connected in an intricate way by 
diagonal fissures, and all are regarded as belonging to a single fissure 
system. The material of the dikes is fine and even-grained sand 
grains, either in the form of sandstone or more commonly indurated 
to a dense hard quartzite. The induration is mainly due to limonite, 
but in some degree is due to muscovite, and to secondary silica. The 
t™Continued from p. 454, Vol. Il. JouRNAL GEOLOGY. 
?Tntrusive Sandstone Dikes in Granite, by WHITMAN Cross. Bull. Geol. Soc. 
Am., Vol. 5, pp. 225-230, pl. 8, 1894. 
