10 NORTH AMERICAN FAUNA. 



The Tres Marias group comprises four islands, San Juanito, Maria 

 Madre, Maria Magdalena, aud Maria Cleofa, arranged in a northwest 

 and southeast direction. Maria Madre, the largest, measures about 

 8 by 15 miles, aud rises over 2,000 feet above the sea. North of this, 

 and separated from it by a channel 4 miles wide and 5 or 6 fathoms 

 deep, is San Juanito, an islet 3 or 4 miles in diameter and about 100 

 feet high. Next southeast of Maria Madre is Maria Magdalena, 

 roughly triangular in outline and 7 or 8 miles across, with its central 

 summit rising to an altitude of about 1,500 feet. A shallow channel 

 8 miles wide separates it from Maria Madre. Southeast of Maria Mag- 

 dalena lies Maria Cleofa, the last of the group. It is irregularly 

 rounded in outline, about 3 miles across, and its altitude is apparently 

 much less than 1,320 feet, as given on the charts. The channel between 

 the two last-named islands is about 12 miles wide and much deeper 

 than the others. 



With the exception of San Juanito, which is nearly flat with a narrow 

 border of low bluffs along the north shore, the islands are mountainous 

 and rise in successive slopes from the shore to the culminating point 

 near the center. The interior of Maria Madre is occupied by a moun- 

 tainous ridge extending almost the entire length of the island, but 

 descending to a gently sloping area near each end. The eastern side 

 of the island has the longer slope, while the westward or seaward face 

 is much more abrupt, thus corresponding with the formation of the 

 mountains parallel to the coast on the adjacent mainland. Both slopes 

 of the island are scored at intervals with canyons which usually descend 

 in a nearly direct line to the sea. Maria Magdalena and Maria Cleofa 

 are occupied by a central mountainous elevation, from which canyons 

 descend in all directions to the sea. The northeastern points of both 

 these islands are low, flat, sandy areas of limited extent, and the west- 

 ern faces are rocky and precipitous. Permanent fresh water is very 

 scarce on all the islands. There are three little streams on Maria 

 Madre, which sink several miles from the sea during the dry season, 

 and one each on Maria Magdalena and Maria Cleofa. 



The relative situation of the islands, with the narrow, shallow chan- 

 nels between them, shows conclusively that at one time they formed a 

 single island at least 45 or 50 miles long, and at a still earlier stage they 

 must have been connected with the mainland. One of the strong- 

 est proofs of this former connection is shown by the correspondence 

 between the fauna and flora. The breaking down of the original island 

 into several smaller ones and the evident continuous encroachment of 

 the sea appear to indicate that the subsidence is still in progress. The 

 country back of the coast on the mainland was, within a comparatively 

 recent period, the scene of great volcanic activity, and the Tres Marias 

 bear evidence of having undergone various oscillations in level. On 

 Maria Madre there are great beds of marine deposits, hundreds of feet 

 above sea level, containing quantities of shells and corals of species 

 now living along the shore. Isabel Island, near the mainland, is of 



