266 SIDNEY POWERS 



or some other mineral to form amygdules. At the base of the flow, 

 rapid chilling also takes place; less glass is developed, but well- 

 crystallized magnetite is found. Alteration, however, soon com- 

 mences in the base of the flow because of the reaction of heated 

 waters on the basalt. 



The glass, characteristic of the top and the bottom of a flow, 

 frequently contains most of the feldspar in laths already formed, 

 showing that the feldspar had commenced to crystallize before the 

 augite. In other cases the glass is accompanied by both augite 

 and feldspar. The glass always has a cloudy appearance. 



Gravitative adjustment takes place in all flows which are 

 sufficiently thick, and which remain hot sufficiently long for a 

 movement of the crystallizing magma to take place without 

 being recorded in flow structure. As in the case of the Palisade 

 sill, olivine tends to form near the base of the flow and in the quickly 

 chilled top. 



Gravitative differentiation is also shown in the relations of the 

 labradorite to augite. The augite settles toward the base of a 

 flow as in the case of a sill, and the feldspar rises. 



The chemistry of the Cape d'Or flows will be treated in a sep- 

 arate paper, but it may be stated here that those basalts show a 

 normal composition, averaging about 52.5 per cent silicia, 14.3 

 per cent alumina. 9.8 per cent lime, 2 . 5 per cent soda, and 1 per 

 cent potash. 



Rosiwal measurements on thin sections from the center of a 

 556-foot flow show a mineralogical composition of 40 per cent 

 plagioclase feldspar, 56.5 per cent augite, and 3.5 per cent iron 

 ores. 



All the basalts show more or less alteration and disintegration 

 except where rapid marine erosion exposes fresh rock. The 

 amygdaloid, even where fresh, is always altered. In the drill- 

 cores at Cape d'Or, the same character of alteration was shown in 

 each amygdaloidal layer. A certain amount of hematite, with 

 limonite, is developed, giving these rocks a reddish color. 



Veins are very common in the dense basalts as well as in ihe 

 amygdaloids. The veins are formed of jasper or quartz, with 

 either reddish (hematite) or greenish (malachite or chlorite) walls. 



