006 MADEIRA. 
Concluding Remarks.—The igneous origin of Madeira, through the 
ejection of its rocks as separate streams of lava or fluid basalt, with 
intervening cinder or fragmentary eruptions, hardly admits of doubt. 
It is also obvious that at least its later eruptions, were, to a great ex- 
tent, subaerial, and continued to a comparatively recent period. Its 
origin has been considered as dating no farther back than the tertiary 
epoch. But of this we have no evidence, and probably none will 
ever be detected. The age of rocks is usually determined either by 
the contained fossils or relative order of superposition : neither kind of 
evidence exists in Madeira. ‘Though we know that the surface rocks 
are comparatively recent, we must also admit that the recent lavas of 
the surface may rest on a continued series of other igneous rocks that 
may count back to an early geological epoch. 
The centre or centres of eruption have not been satisfactorily ascer- 
tained. ‘The Corral is considered by some writers one of the craters, 
and it has a very strong resemblance to Kilauea in the Sandwich 
Islands ; for there are, at Kilauea, the same abrupt walls, regularly 
stratified from top to bottom, and as free from scoria, and it would 
be necessary only to open a gorge from the south extremity to the sea 
to complete the resemblance. But other examinations may be required 
to determine with certainty whether this is its real origin, or whether 
it has resulted from a subsidence attending an eruption from some 
other centre. 
The recent elevation of the island indicated by the San Vincente 
limestone appears to have amounted to two thousand five hundred 
feet. From the limestone of Canigal, we should infer that the rise in 
that part since its formation was eighty or one hundred feet, and that 
it was unequal in amount on the two sides of the valley. 
The islands of Porto Santo and the Desertas belong to the Madeira 
Group, but were not examined by us. 
It is interesting to observe that the trend of the island of Madeira, 
is the same with that of a line from the Desertas through Madeira, it 
varying little from N. 66° W.; while the trend of Porto Santo is 
N. 42° E., or within eighteen degrees of being at right angles with 
the Madeira line. 
