22 THE VOYAGE OF H.M.S. CHALLENGER. 



Among our specimens there were fragments of the limestone taken from the raised 

 beach he describes. 1 We refer to his book on Volcanic Islands for the details 

 relating to the changes which have affected this calcareous rock in contact with the 

 overlying volcanic products. Doelter 2 remarks that he was able to trace this altera- 

 tion only on a layer of 10 inches, at the contact of the Limestone and lava. The 

 limestone has become granular, and some of its grains are rather large. These are the 

 only phenomena of contact observed by Professor Doelter at San Jago. Other observa- 

 tions on the same subject made by Darwin must, according to Doelter, be explained in 

 another way. 



A specimen of limestone from this raised beach has been collected at the contact of the 

 lava. This calcareous rock is massive ; the layer near the lava is opal blue, and the 

 grains are somewhat larger ; the other part of the specimen is brownish. Calcareous 

 grains and small fragments of volcanic material are cemented by infiltrated calcite. 

 Near the zone of contact the grains are of a deeper blue, but the saccharoid structure is 

 not clearly shown. The calcite of this thin zone of contact effervesces with hydrochloric 

 acid, leaving a residue composed of organic matter ; it yields only a trace of magnesia. 

 The white and bluish grains are fragments of organisms, as can be ascertained by 

 microscopic examination. Under the microscope it is seen that the organic structure is 

 not entirely destroyed ; the sections showing this are less transparent than those of 

 infiltrated calcite, and they are speckled with brown and bordered by a yellowish zone. 

 The secondary calcite is clear and crystalline, showing the rhombohedral cleavage 

 characteristic of this species. The small volcanic fragments embedded in this limestone 

 are splinters of basalt, palagonite, augite, olivine, hornblende, and biotite. They are 

 isolated and entirely surrounded by infiltrated limestone. Microscopic concretions of 

 iron and manganese oxide are also to be seen. 



Another specimen from this raised beach is very like that we have just described, but 

 it contains more volcanic fragments ; among these are found all the rocks and minerals 

 above mentioned. Augite is specially abundant. As in the former case, saccharoid 

 structure cannot be observed. The details of the organic structure are not washed out, 

 and the hemitropic striae following — ^ R are never seen. 



We shall mention, in conclusion, a specimen of Limestone which covers the lava on the 

 coast near Porto Praya, and which ought to be considered as a stalactitic deposit. This 

 specimen is brownish yellow, and is formed by the superposition of more or less folded 

 and slightly adherent lamellae. Calcite has crystallised in the cavities, and some small 

 elongated scalenohedric crystals can be recognised. This coating contains compact black 

 volcanic splinters two or three centimetres in length ; these are glassy fragments 

 passing to palagonite. Darwin observed these inclusions, and compared them rightly 

 to the palagonite found by him in the Galapogos Islands. Under the microscope no 



1 Darwin, loc. cit., p. 3. » Doelter, loc. cit., pp. 45, 191. 



