TY  - RPRT
TI  - The Helderberg Group and the position of the Silurian-Devonian boundary in North America
AU  - J.M. Berdan
PY  - 1964
DO  - 10.3133/b1180b
UR  - https://pubs.usgs.gov/publication/b1180B
ID  - b1180B
ER  -