MINERALOGY: G. P. MERRILL 
429 
tected, as it were, from another crossing over. This relation is illus- 
trated in the diagram (fig. 7). If crossing over occurs at some point 
between two pairs of factors a and b indicated on the diagram, the next 
crossing over point of the chromosome would, if it occurred at all, lie 
at some distance away, rather than near by. This possibihty can be 
tested in a case where several known points are present in a pair of 
chromosomes. If in such a case we determine how often crossing over 
occurs between A and a in general and then determine how often it 
occurs between A and a in those cases where it is known to have taken 
place between a and b, we find an enormous decrease in the number 
of times it occurs between A and a when at the same time it has 
occurred between a and b. In general one may say that crossing over 
at any level interferes with crossing over in the region of each side of 
that level. 
Conclusions. The chromosomes not only furnish a mechanistic ex- 
planation of Mendelian heredity, but in the case of Non-disjunction 
and in the case of the point by point correspondence between the Link- 
age Groups and the chromosomes, furnish a verifiable explanation of the 
results. In the case of Crossing-over and of Interference the chromo- 
somes give us the only objective explanation of the results that has 
been as yet offered. 
RESEARCHES ON THE CHEMICAL AND MINERALOGICAL 
COMPOSITION OF METEORITES 
By George P. Merrill 
DEPARTMENT OF GEOLOGY. UNITED STATES NATIONAL MUSEUM. WASHINGTON 
Preiented to the Academy. June 1 . 1915 
The primary motive of these investigations was to test the authentic- 
ity of numerous reported occurrences of certain minor constituents, 
such as antimony, arsenic, gold, lead, tin, tungsten, uranium, zinc, etc., 
and incidentally to formulate the analyses in such a way that the results 
might be made comparable with those of terrestrial rocks. Upwards 
of twenty meteorites were subjected to searching chemical analyses, 
with the particular end in view stated above. The results were in part 
confirmatory and in part contradictory. In none of the samples tested, 
either metallic or stony, could any traces be discovered of antimony, 
arsenic, gold, lead, tin, tungsten, uranium, or zinc. The presence in 
traces of platinum, palladium, iridium, ruthenium, and vanadium was, 
however, proved beyond apparent question, ruthenium being noted for 
the first time, and vanadium having previously been reported but once. 
