GEOLOGY OF THE WHITE SANDS OF NEW MEXICO 113 
so-called tripoli described by the writer some years since. 
There is no reason to alter the opinion then expressed that this 
fine-grained scale-like deposit is the result of the attrition of 
the floating pumice which forms the surface of the deposit. In 
fact, in several other places in the Rio Grande valley similar beds 
on a smaller scale have been encountered, and in each case the 
material could be traced directly to the acid scoria of the 
period of trachite eruption. 
To the southeast we pass to the celebrated Carthage coal 
belt, at which point a collection of Cretaceous fossils was made, 
but, as they were not found in immediate connection with the 
coal beds, it is impossible to decide what is the age of the coal 
upon that basis alone. However, a little farther south in the 
vicinity of Engle and East of the Caballo Mountains fossils of 
the Laramie age seem to prove that the coal fields at this point 
are of that period. 
At San Marcial and at frequent intervals down the valley are 
basaltic cones which have broken through the Tertiary gravels 
and marls and supplied the material for the sheets of lava so 
characteristic of the entire territory. It is easy to see that they 
follow in a general way axes of weakness extending north and 
south, but it is not so easy to determine the reason for a sudden 
return to highly basic conditions after a gradual increase in 
acidity in the volcanic flows of the territory. As the writer has 
shown in several papers, the sequence is from an augite-andesite 
or diabase through trachite and pitchstone and obsidian to rhy- 
olite. The soda-syenite and phonolite may perhaps form a 
transition from the andesite, though the occurrence of the soda 
series is less general 
It suggests itself to the writer that the serial arrangement 
is to be attributed to an invasion of the silicious crust by the 
internal heat, and that progressively less of the deeper material 
was involved in these flows until it may be said that that chapter 
of igneous activity was closed by the rhyolite eruptions. Long 
after, perhaps as a result of the differential strain of glaciation 
and its attendant shifting of the axes of rigidity of the crust, 
