134 The American Geologist. March, looi. 
dikes as a general rule all have a vertical dip, and a northwest- 
southeast trend, thereby invariably cutting the gneisses and 
schists at a considerable angle. In some instances the large 
dikes, owing to their slow weathering, have given rise to low, 
flat ridges whose sides are usually strewn with innumerable 
rounded bowlders, locally called "nigger-heads." 
All of the trap dikes throughout the Crystalline area ap- 
pear to be of the same age and are formed of similar rock ma- 
terial. They are evidently of a comparatively recent geolog- 
ical age as is shown by their undisturbed condition. They 
rarely ever reveal any evidence of shearing or any other in- 
dication of a general earth movement. Along the southern 
margin of the Crystalline area between Alacon and Milledge- 
ville at a point on the Georgia railroad, near James' Station, 
a dike comes in contact with clay beds which have been classed 
by Dr. Geo. E. Ladd as Tertiary beds.* The exposure here, 
however, is limited and gives no satisfactory evidence as to the 
relation of the dike to the Tertiary clays. A further study of 
the dikes along the above contact will probably demonstrate 
that they are of Jura-Trias age, and belong to the same system 
as the trap dikes of the Carolinas and Virginia. 
The rocks of the dikes are quite compact, fine-grained and 
of a dark-gray, or almost black color. They are typical dia- 
bases made up of plagioclase and augite with olivine and mag- 
netite as the chief accessory minerals. The plagioclase occurs 
in the form of long, slender, lath-shaped crystals, which are 
frequently enveloped in large irregular plates of augite, thus 
exhibiting a beautiful ophitic structure. 
THE PLAN OF THE EARTH AND ITS CAUSES 
By J. W. Gregory, D.Sc, Melbourne, Aus. 
• {Continued from p. 119.) 
THE EARTH A GEOID. 
But it may be said this tetrahedral theory is impossible, be- 
cause we know from our elementary text-books that the earth 
is not terahedral, but is an oblate-spheroid — that is to say, a 
sphere slightly flattened at the poles. 
The oblate spheroid is no doubt the form that rotation 
would have caused the earth to assume as it solidified, if the 
• The American Geologist, vol. xxxiii, pp. 248. 
