Geol.— Vol. I.] SMITHS A XTA CAVA UNA ISLAND. 5 I 



microscopically. While the slide of the rock showed no 

 fragmental material, this powder showed a very few scattered 

 mineral fragments here and there, but even in this con- 

 centrated form they are not so numerous as in the slides of 

 the shaly specimens. Aside from these the residue is wholly 

 isotropic and consists largely of the remains of diatoms, 

 wdth occasional Radiolaria. It is interesting to note, in com- 

 paring the chemical and microscopical characters of the 

 rock, what a small percentage of organic remains is neces- 

 sary to give a slide the appearance of being well filled with 

 them. 



The results thus show that the rock is a magnesian lime- 

 stone. Without doubt the carbonates are original and not 

 secondary, their source probably being the sea-water in 

 which the organic forms, were laid down. 



C. SEDIMENTARY DEPOSITS. 



Besides the beds at the isthmus, two minor deposits of 

 sedimentary material were found, both within the main area 

 of the andesite. One occurs in a saddle on the principal 

 ridge to the west of Orizaba, at a distance of not more than 

 a mile; the other, shown by a dotted surface on the map, is 

 on the lower slopes of the andesite in the Little Harbor 

 region. 



The first mentioned is only a small deposit in the lowest 

 part of the saddle, extending not more than five or six feet 

 up the slope on either side. The rock which forms the 

 deposit proper is more or less open and is composed almost 

 wholly of coarse shell fragments roughly cemented with 

 secondary calcite. These fragments, though too small for 

 a specific determination, appear to be the remains of large 

 pectens. In addition to this, there were seen a number of 

 rolled pebbles of andesite and porphyrite, and several large 

 blocks of a white earthy material, all of which appear to 

 have been deposited here. It is possible, however, that they 

 were brought here by the Indians who formerly occupied 

 the island, since there is unmistakable evidence that this 



