76 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



are known only from Maria Madre. In addition to a species of Cedrela, 

 a giant Agave, and a native cotton mentioned by authors and still un- 

 identified, are many other plants still awaiting collection and study. 

 Altogether these islands offer a most interesting subject for field research. 

 The job calls for a botanist of ethnographic and geologic background 

 in insular residence throughout the summer and fall, at least. 



San Juanito IsIand.The Tres Marias Islands are actually four. 

 The odd one is San Juanito, the northwesternmost of the group, lying 

 across a narrow channel from Maria Madre. It is the smallest, being 

 about 5 kilometers long, 3 kilometers wide, 15 square kilometers in area, 

 and about 300 meters in elevation. Nelson and Goldman are apparently 

 the only biologists who have visited it and the former gives the follow- 

 ing short description (1899:10, 12), "San Juanito which is nearly flat 

 with a narrow border of low bluffs along the northern shore — . On San 

 Juanito the vegetation is largely made up of bushes and scrubby trees 

 8 to 15 feet high, with many Agaves on the sandy southern end. Agaves 

 are very numerous also on the northern end of Maria Madre." There 

 are no known plant collections and it apparently has never been visited 

 by botanists, the accounts of other expeditions do not mention any land- 

 ings there. 



Maria Madre Island is the largest of the Tres Marias group, 

 21°35' N. by 106°40' W. It is approximately 20 kilometers long, 10 

 kiolmeters wide, and 200 square kilometers in area. The peak of the 

 island is 616 meters above sea level. 



The land forms consist of narrow beaches, cliffs, canyons, mesas, hill 

 slope and ridge. Arroyo Hondo is a notable intermittent water course 

 originating near the central peak and descending northwestward through 

 a deep canyon to discharge water into the sea during the rainy season. 

 In the past it has been difficult of access by land. Hanna (1926:67-71) 

 found the island to consist of a central pediment of granite with Plio- 

 cene marine sediments of chert, limestone, and sandstone lapped upon 

 it to near the top of the peak. ''During a portion of Pliocene time large 

 coral reefs existed around this old land mass and large blocks of the 

 fossiliferous material, firmly cemented, having fallen from the exposures 

 and have rolled indiscriminately far out into the forest." The island 

 appears to have had its inception in Miocene times, if not before, and 

 grew in area during the Pliocene. Maximum size was probably attained 

 during one of the glacial periods when sea levels were universally lower. 



What is known of the vegetation has been described above. As with 

 nearly all of the Eastern Pacific islands, very little attention has been 



