REVIEWS 507 
ing away of the moon, may not have existed until the end of the Paleozoic, 
with the result that, granting the present volume of sea-water, a universal 
ocean, averaging 10,400 feet deep, previously covered the whole earth. 
How, under such conditions, there came to be any sediments in the Appa- 
lachian geosyncline from a land ‘‘of continental proportions” the author 
does not attempt to explain. 
Again, on page 31, the basic nature of the Hawaiian lavas is explained 
as a consequence of the removal of the lighter acid crust from the ocean 
basins to make the moon. It is easy to see how on this hypothesis the 
author would account for the enormous basaltic outpourings which have 
taken place over the surface of the continents, but it would be interesting 
to have his explanation of the fact that in Washington’s tables of chemical 
analyses of igneous rocks the only analysis reported from Kerguelen Island 
gives a phonolite with 58.2 per cent. of silica and 12.0 of alkalies; the 
only one from Gough’s Island a trachyte obsidian with 61.2 per cent. silica 
and 12.4 of alkalies; the only two from Ascension Island are trachyte 
and obsidian, the former with 71.0 per cent. of silica, the latter with 72.7. 
The second part of the article, entitled ‘The Volcanic Problem,” does 
not seem to have any very clear or close relation to the first part on the 
‘Place of Origin of the Moon.” 
JosEPH BARRELL 
Essentials of Crystallography. By E. H. Kraus. Ann Arbor, 
Michi>Geo. Wahrs Pp. x+-162; 427 figs. im text. 
In the words of the preface, this book is ‘‘intended for beginners and 
aims to present the essential features of geometrical crystallography from 
a standpoint which combines the ideas of symmetry with those of holo- 
hedrism, hemihedrism, etc.’”’ All the possible crystal forms are discussed 
with the important classes pointed out. 
A bibliography of forty-one titles of important reference books and 
articles is at the beginning of the book. This is followed by a sixteen- 
page general discussion of the properties of crystals, their arrangement 
into systems, the symbols used, the symmetry, and tractional forms. The 
systems are then taken up in order, beginning with the cubic and following 
through to the triclinic. The relations of axes, symmetry, and possible 
classes are taken up with considerable care in each system. 
Compound crystals, with a full discussion of the laws of twinning, forms 
the last chapter, and is followed by a concise tabular classification of the 
classes, showing at a glance the planes, axes, and centers of symmetry, 
the possible forms, and the representatives, when known, of each class. 
