BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 
Volleg95 RR: 18-34, .PES. 1-5 d DECEMBER 31, 1897 
GREAT CHANGES OF LEVEL IN MEXICO AND THE INTER- 
OCHANIC CONNECTIONS 
BY J. W. SPENCER 
(Read before the Society August 10, 1897) 
CONTENTS 
Page 
IIAROCIUICHIOI 3 as bia 65.6. co Ce Acres Ean OIRO a ienstee ey nays ao eet ec eeree eas earn 13 
Halivcicaleteatumessor MMIESACOM ee acc ect shone Hele tials eveletane ae be ene clolepdemeses 14 
Coastalfplain otyeasterm Mexico. . 2225.25 s05 eae secede sen sence. aoe. 14 
Atlantic coastal plain of the Tehuantepec isthmus...................... 14 
Pacific coastal plain of the Tehuantepec isthmus........... ...--..+---- 15 
PINCUS VS tawemevaus ey rar aisversiscs ls. frase vale soay bun aeneneats aielae ein ieare lee snegeee os 15 
Mountain region of the Tehuantepec isthmus........... ............-.-- 15 
Declivity and terraces of the valleys descending from the high plateaus...... 16 
Geological basement of Mexican topography...... ........---eeeees ee eens 20 
Older geology of the Tehuantepec isthmus..........-......2.2-200ceeee cee 21 
Later formations of the Pacific coastal plain of Tehuantepec ................ 22, 
Later formations of the Atlantic coastal plain of Tehuantepec.........-..... 23 
Coatzacoal Cospronmationer: <-\.1. sqm heivteteeaysrisic lone etter euehelstan earn, 23 
Character, extent, and relations...........-... .ceeeceee sc eeeeee ees 23 
SLUT DR UIN MACS prays ta encase. so altiebestcnnieeieneve eran bel aexeeneicuet onstrates sere eeeats 24 
Some unassigned deposits of the coastal plain........................... 25 
Lafayette and Columbia formations in Mexico................+......02.005: 25 
In northern Mexico...... a Lee dae NAS Sines ah cite ame Re mearenate a cea Le 25 
limacentual-eastern MexicOyin. 62. <u sewele adldchtocmesaietae cine. eile dies 6 = 26 
On the Gulf side of the Tehuantepec isthmus .......................4.. 28 
Terraces on the coastal plains.......... eda stoa ets ais BS Sica to NRA ie Aran ce 30 
Geolocicalcanalof'Chivelac. 5. 2... 65 see sccm see emnweeaneceres heeanh wc 31 
Biological evidence of interoceanic connection....................... sees 32 
Siimmary ancl ConclhsOnssyahanens Bepecoosousosoncoo nasaoaseoopeencooeds 32 
INTRODUCTION 
In the studies relating to the submarine valleys and plateaus of the 
West Indian waters which resulted in the contribution entitled ‘“‘ Recon- 
IlI—Butu. Gro. Soc. Am., Vor. 9, 1897 (13) 
