74 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 13 



TRES MARIAS ISLANDS 



The Tres Marias Islands are out on the steep edge of the con- 

 tinental shelf about sixty to seventy miles off the mainland shore of 

 Nayarit, opposite the port of San Bias. They appear to have been in 

 existence since sometime in the Pliocene and in part much earlier. They 

 are composed of several kinds of both igneous and sedimentary rocks. 

 The Miocene and Pliocene marine formations on Maria Madre and the 

 Mesozoic rocks on Maria Magdalena indicate local differential faulting. 

 To Nelson (1899:9-11) the position of the islands upon the continental 

 shelf and the similarity of the vertebrate animals to those of the main- 

 land indicated a Quaternary land bridge. However, the seven species of 

 endemic mammals, the 24 species and subspecies of endemic birds and 

 an endemic reptile, as well as the 21 species of endemic plants known 

 from the islands, strongly indicate a considerable period of insular de- 

 velopment. A local insular biota was apparently well developed before 

 the Quaternary land bridge, so that aggressive mainland adventives 

 would have had to compete to establish themselves. 



Soundings show a rather narrow submarine ridge extending out from 

 Punta Mita, Nayarit, toward the Tres Marias and in line with their 

 axis. The deepest sounding along this line shows 70 fathoms (ca, 30 m), 

 with greater depths to port and starboard. Glaciation is thought to have 

 lowered sea levels from 80-100 m (Zeuner, 1945:248), which is suf- 

 ficient to cause peninsulation in this case. Probability of the land bridge 

 hypothesis is also dependent upon local epirogenic movements and a 

 study of geology still in waiting. 



The climate of the Tres Marias is subhumid, tropical, maritime, 

 and equable with a binary pattern of seasons; rainy summer, and arid 

 spring. The maritime influence appears to ameliorate the long spring 

 dry season. (Grayson 1871:267) *'In the dry season heavy dews are 

 frequent, the drops of which I have often seen the birds sipping, for 

 want of other means of quenching their thirst, there being but few 

 ojas de agua (springs)." Summer rains are convectional in type, the 

 "chubasco" storms of wind-driven rain being common and making navi- 

 gation for small boats hazardous. Hurricanes in the fall rarely swing 

 in across the islands from the southwest (Schiaffino, 1939). No rain- 

 fall data are available for the islands but precipitation should be ap- 

 proximately that of San Bias on the mainland, which has an annual 

 average of 58.5 inches. The annual average temperature at San Bias is 

 24° C, with no record of frost (1939). 



The natural vegetation of the islands is a subhumid tropical drought 



