1845.] HUNT ON ST. Mary's island — Azores. 39 



3. On the Geology of the Island of St, Mary's, one of the Azores. 

 By T. Carew Hunt, Esq., Her Majestj^'s Consul for the Azores. 



The island of St. Mary's, the easternmost of the Azores, is about 

 seven miles in length from east to west, and five in breadth from 

 north to south, and is divided naturally into a plain and an elevated 

 district, which, taken together, mark its aspect from the sea. The 

 plain occupies about one-third part on the west side of the island, 

 the other part being formed of an elevated ridge running from north- 

 west to south-east, which rises to a double peak (Pico Alto) 1889 

 feet above the sea, and of which the sides decline on the north, east 

 and south to mural cliffs of considerable height. 



The base appears to be a dark blue compact basalt, containing 

 minute crystals of olivine. This basalt has in some parts assumed 

 a contorted columnar structure and near the town of Villa do Porto 

 a distinct prismatic character. Above this bed is a coarse red 

 wacke, passing to a cellular brown amygdaloid, its softer parts 

 thickly imbedded with splendent crystals of augite, and the cells of 

 the harder mostly filled with arragonite in three varieties of cry- 

 stallization, and carbonate of lime. In both are occasional im- 

 perfect crystals of hornblende. This would seem to be the second 

 bed of the island, and to it succeed deposits of marine shells of va- 

 riable thickness, in which the most conspicuous shells are the lesser 

 and greater scallop, mixed in parts with an arenaceous cement and 

 rounded nodules of basalt, breccia and amygdaloid, and interstrati- 

 fied with films and layers of semicrystalline carbonate of lime. This 

 calcareous bed is of extreme hardness, except where its sections have 

 been exposed to and rendered brittle by the spray of the sea. On 

 the surface is a strong grayish argillaceous earth mixed and covered 

 with small decomposing basaltic pellets of a concentric laminated 

 structure, which, with beds of tufa of different degrees of coarseness, 

 completes the constitution of the plain district. 



The lower beds of the second or elevated district (marked B) are 

 probably a continuation of those described, overlaid by the mass of 

 brown, compact, and sometimes amygdaloidal porphyry, which has 

 formed the chain of heights. At St. Lorenzo is a detached ca- 

 vernous rock of basalt veined with carbonate of lime, whose inte- 

 rior is filled with stalactites, — a variation of base which occurs in 

 other parts of the island, and extends to the Formigas rocks, distant 

 twenty miles to the north-east. 



The general conclusion will perhaps be, that the two lower beds 

 of basalt and amygdaloid were of successive submarine formation ; 

 that the lapse of many ages allowed them to be covered with a thick 

 layer of marine shells and sand ; that a new eruption covered this 

 layer and partly fused it ; and that the island was completed by the 

 formation of Pico Alto. It will be a question, whether its emer- 

 gence preceded, accompanied or followed the last convulsion, and 

 also at what relative period a small hill called the Pico do Facho, 

 near Villa do Porto (connected apparently with some neighbouring 

 basaltic dykes), was formed. 



