856 THE JOURNAL OF GEOLOGY. 
As to the forms of laccolites he does not believe in limiting the 
term to those intrusive bodies only that occupy a perfectly regular 
position with regard to stratification planes, but would apply it to any 
intrusive body where the expansion of the body has taken place from 
a plane even approximately parallel to the bedding. In horizontal 
strata the lifting of the load by the intrusive force may be taken as the 
prime essential. The deviations from the type forms from accidental 
" causes are many. With regard to the origin of laccolites Cross cites a 
number of facts which demonstrate that the horizon occupied by intru- 
sive magmas are not determined by relative densities of the intruding 
lavas and of the invaded strata, as suggested by Gilbert, and assuming 
eruptive energy such as exists in active volcanoes he concludes in the 
words of James D. Dana that “‘no other cause could be needed for a 
flow to the surface in case of an open channel, or for a flow to any level 
in the strata at which a fissure might terminate ; and this is true whether 
the lava be light or heavy.” jee ale 
Petrology for Students. An introduction to the study of rocks 
under the miscroscope. By ALFRED Harker. Published 
by Macmillan & Co., New York, 1895. Price $2. 
As the author states in the preface this text-book is prepared espe- 
cially for English students, nevertheless it will be found very useful 
for those beginning the study of petrography in this country, who 
wish a text-book written in English. No systematic account of the 
crystallographic and optical properties of minerals has been attempted, 
and for such information the student is referred to the translation of 
Professor Rosenbusch’s volume on the rock-making minerals. But as 
an introduction to the study of the rocks themselves a number of use- 
ful observations of a general nature are presented upon the characters 
of minerals in thin section, and especially the latest methods of dis- 
tinguishing the different varieties of feldspar. In treating so complex 
a subject as the optical properties of minerals in thin sections in such 
a condensed manner it is doubtful whether the author can meet the 
wants of a beginner. It serves, however, as a form of definition of the 
terms used throughout the book. It would seem that in neglecting 
the use of those methods of determination based on the optical phe- 
nomena observed with converging polarized light the author need- 
lessly weakens the processes of petrographical diagnosis. 
In his remarks upon the examination of rock sections the author 
