Geology, and Climate of the Island of Madeira: 347 



he examines the ravines in the centre of the island, or the 

 cliffs on the coasts which the action of the sea has exposed to 

 his view, the rocks of which the whole mass of the land is com- 

 l)0sed are found to be of volcanic origin. Some marine for- 

 mations there are, but in a comparatively small proportion. 

 Of the volcanic products, some have been thrown out in the 

 form of fused lavas, others have been projected into the air, 

 and others deposited in a state of aqueous suspension. With- 

 out giving any detailed description or enumeration of the va- 

 rious rocks, I may here state some of the more interesting 

 points in the geological appearances of the island. 



I. Volcanic Formations, 



Basalts.— The igneous rocks are all of the basaltic class. 

 They present great variety of structure, according to the tem- 

 perature and pressure to which they have been exposed, their 

 contact with atmospheric aii* or other gases, and other influ- 

 ences to which they have been subjected. The basalt is, for 

 the most part, compact ; but much of it is more or less vesi- 

 cular, and some also scoriaceous. The compact basalts are 

 sometimes, in structure, altogether homogeneous, but fre- 

 quently contain a variety of minerals, especially crystals of 

 olivine, which occurs in great abundance. In the vesicular 

 basalt, the gaseous interstices, while compressed by the force 

 of gravity, are also generally elongated in the direction of the 

 current of the fluid rock. Observation of this is important in 

 reference to the origin of these currents. The scoriaceous and 

 cellular basalt occurs wherever the lava has been much ex- 

 posed to the air, as in the smaller streams, and in the upper 

 and outer surfaces of the great deposits. In such situations 



TV. 



as well as of the English, of whom (including; the strangers who reside in 

 Funchal during the winter) there are about 500 in the island It is much to 

 be wished, that, when circumstances permit, tliis undertaking should be re- 

 sumed, for (apart from the amusement that the literary department Avould 

 afford in a place where there are not many sources of occupation to the 

 strangers) there ai-e few foreign stations from which, on many points in na- 

 tural history, horticulture, and other subjects, a greater mass of valuable ob- 

 servations would be readily contributed, if a loc^ modium of scientific coni- 

 municntioa were established. 



