140 The American Geologist. September, i896 
'J'he beds will ultimately have some economic importance as 
abraiuhnits or filters. Probabl}'^ b}' grinding between burrs 
and sorting by a cetrifugal a good quantity of polishing pow- 
der could l)e obtained, though lacking in the " tooth " of gen- 
uine " tri[)oli." For filtration and absorbents the material is 
admirably suited. The relative freedom from iron and impur- 
ities is a very important element. 
Geologically the occurrence is especially interesting, as the 
material in this form seems to be unique. 
For specimens and information respecting these beds the 
writer is indebted to Mr. Geo. H. Thwaitesof Socorro, and for 
laboratory facilities and the included analyses to professor 
W. A Seamon, director of the New Mexican school of mines. 
EXPLANATION OF PLATES IV AND V. 
Plate IV. General view in the Rio Grande valley looking north. The 
"tripoli" beds in the foreground. Three ranges appear in the distance 
on the opposite or west side of the river. They are the following, pass- 
ing from left to right: The Socorro mountains, the Limitar range, the 
Ladronnes. 
Plate V. Nearer view of the "tripoli" showing the upper scoria-crag 
and middle "tripoli" deposits. 
Photographed by J. E. Smith, of Socorro. 
PAL/EONTOLOGY AND THE BIOGENETIC LAW.* 
By Karl von Zittel. 
PalfBontologv has long since ceased to place itself exclusive- 
ly at the service of geology as the study of characteristic fos- 
sils. It has gradually grown into an independent branch of 
the biological sciences, and claims a share in all their move- 
ments and tendencies. The conclusive establishment of the 
doctrine of descent has evoked the most powerful revolution 
in descriptive natural history, influencing and transforming 
its whole method of research. No large palfeontological work 
of to-day contents itself with the description of new forms, 
the comparison of them with those already known, and the 
arrangement of them in systematic order. To determine the 
generic relationships, the ancestry, the modification, and the 
further development, in short, the race-history or phylogeny 
of the organisms under consideration, is now regarded as es- 
sential, by many indeed as the chief end of pah>3ontology. 
*A paper read before the International Congress of Geologists, 189J:. 
