266 RECENT FORMATIONS. 



gists. As a curious illustration, we may notice the island 

 of Sombrero, one of the West India group, which is almost 

 entirely of organic origin. This islet, so called from its re- 

 semblance to a " sombrero," or low-crowned Spanish hat, 

 and situated about 130 miles east of Porto Rico, is about 

 two and a half miles long, one-half to three-fourths of a 

 mile wide, and rises from 20 to 30 feet above the level of 

 the ocean. It is a barren rock, and appears to be entirely 

 composed of the rich phosphatic mineral known in com- 

 merce as Sombrero guano. This substance imbeds numer- 

 ous bones of turtles and other marine animals ; and from 

 its composition, which resembles bones deprived of their 

 cartilage, it has been supposed (with every degree of proba- 

 bility) that the island was once a shoal swarming with 

 turtles and other vertebrate animals, whose accumulated 

 remains of ages have been cemented together, and gradually 

 elevated above the ocean-level to the present position of the 

 island. The history of such an accumulation is interesting 

 not only on its own account as a Recent Formation, but 

 from the light which it throws on the origin of osseous 

 breccias belonging to earlier epochs. 



Besides these more obvious operations of plants and 

 animals, there are others equally extensive though less 

 apparent. The very lowest forms of life organisms that 

 stand, as it were, between the confines of the vegetable and 

 animal kingdoms, and which require the aid of the micro- 

 scope to reveal their forms are equally busy, in equally 

 inconceivable numbers, in accumulating their calcareous 

 and siliceous remains. The calcareous oaze or mud of the 

 deep seas, extending for thousands of miles, and of unknown 

 thickness, is, as we have seen under the head of Marine 

 formations, chiefly composed of the minute shells of fora- 

 minifera ; and many siliceous muds in lakes, estuaries, and 

 even in the deeper waters, are composed in like manner 



