D 
A Fossil Flora from Pagan, Mariana Islands 1 
F. R. Fosberg 2 and Gilbert Corwin 3 
Pyroclastic deposits on Pagan, northern 
Mariana Islands, contain numerous plant im- 
pressions. During geologic investigations in 
1954, ten collections were made for subse- 
quent identification and study. Seventeen 
plant species, all of which now live in the 
Marianas, have been distinguished. 
The geologic investigations of Pagan were 
undertaken by a field party of the Office of the 
Engineer, U. S. Army Forces, Far East, staffed 
by members of the U. S. Geological Survey. 
L. D. Bonham of the field party first noted 
the plant fossils. Gilbert Corwin, with the aid 
of M. J. Terman, also of the field party, and 
Santiago V. Castro, a resident on the island, 
collected samples and made field studies of 
the fossil localities. Fosberg has identified, 
studied, and described the plant impressions. 
location and geography 
Pagan Island is near the center and is the 
largest of the northern Mariana group. It lies 
between latitudes 18°01' and 18°11' North 
and between longitudes 145°41 / and 145°49' 
East, about 280 miles north of Guam and 
1,100 miles south-southeast of Tokyo, Japan 
(Fig. 1). 
The northern Mariana Islands form a chain 
nearly 300 miles long that consists of the 
summits of large volcanoes rising from ocean 
depths of as much as 6,000 feet. Uracas 
(Farallon de Pajaros) at the north end is one 
of the most active volcanoes of the western 
Pacific. Pagan, Asuncion, Agrigan, and Gu- 
guan have had eruptions since 1900 (Tana- 
kadate, 1940). The islands are generally small, 
relatively high, and rugged. The maximum 
1 Publication authorized by the Director, U. S. 
Geological Survey. Manuscript received March 4, 1957. 
2 Botanist, U. S. Geological Survey, Washington 
25, D. C. 
3 Geologist, U. S. Geological Survey, Tokyo. 
elevation of 3,136 feet is at the summit ot 
Agrigan Volcano. 
Pagan consists of two active volcanic cen- 
ters located within broad circular depressions 
(calderas) that are connected by a high rugged 
isthmus. It has an area of 18.4 square miles 
and a greatest elevation of 1,890 feet near the 
south end of the isthmus. 
The northern caldera is 314 miles in diam- 
eter and has one large central cone, Mt. 
Pagan, that has an elevation of 1,855 feet. 
Relatively recent basalt flows are extensive 
and form broad plains north, east, and south 
of the volcano; ash is concentrated to the 
west (leeward). 
The southern caldera is about 1 Vi miles 
across. South Volcano within it consists of 
four coalescing cones, three of which have 
broad steep-sided craters. Rough lava plains 
bound the volcano on the north, east, and 
south. To the west the volcano slopes directly 
to the ocean. 
The isthmus ranges in width from about Vi 
mile at the north end to 1 mile at the south 
end. Northward along the crest of the isthmus, 
the peaks become successively lower. The 
caldera backslopes north of the isthmus are 
dissected by numerous valleys directed away 
from the rim of the northern caldera. 
The volcanic rocks have been divided into 
two major groups: pre-caldera and post- 
caldera. Both consist of flows and pyroclastic 
rocks. 
The older, pre-caldera group is well ex- 
posed in the caldera walls, along the coasts 
of the isthmus and southern end of the island, 
and in old sea cliff remnants north and north- 
east of Mt. Pagan (Fig. 1). The lavas and 
pyroclastic rocks are products of eruptions by 
at least four major volcanoes and a number of 
minor vents aligned along or near the axis of 
3 
