20 Johnson and Warren — Geology of Rhode Island. 



stituting by volume upwards of 85 per cent of the solid rock, 

 were in the act of solidifying or had already done so. 

 Whether the feldspar did separate first or not, its final outlines 

 were determined by the surrounding minerals. It is to be 

 noted that the feldspar exists only as the pure phase-feldspar — 

 and is not seen in any relation to the other minerals which can 

 be interpreted as a eutectic structure. Feldspar is not seen 

 within the body of the ore. 



The ore, although it consists of three distinct minerals, so 

 far as its textural relations to the other minerals are concerned, 

 behaves as a textural unit and will be looked upon as such. 

 Within the olivine are included ore grains of some size not to 

 mention the finer grained inclusions which may be in part 

 secondary. Besides inclosing the olivine collectively as a 

 matrix, the ore contains many single crystals of olivine inclosed 

 in the body of its substance. Nevertheless the ore and the 

 olivine have as a whole separated from each other and the 

 olivine has clearly dominated the situation, forcing the ore 

 into the role of a containing matrix. Here again there is 

 nothing that suggests a eutectic structure. 



The constituent minerals of the ore matrix also bear very 

 interesting relations to one another. The spinel, or a part of 

 it, and a little of the ilmenite crystallized first. The re- 

 mainder consolidated as an intimate intergrowth of ilmenite 

 and magnetite in which the ilmenite acted as the host. To 

 those familiar with the solidification of alloys or salts, this 

 intergrowth suggests by its appearance a eutectic mixture of 

 ilmenite and magnetite. The exact quantity of spinel which 

 exists as a pure phase has not been estimated, but it is likely 

 that some of the spinel molecule remains in isomorphous rela- 

 tions with the magnetite. Whether the intergrowth is a true 

 eutectic mixture or not can of course only be told by experi- 

 ment, and so far as the writer is aware, no experimental study 

 of the system ilmenite-magnetite has ever been made. It 

 seems likely that such a study w T ould throw not a little light 

 on the character of " titaniferous magnetites " and steps have 

 been taken toward carrying one out. It may be remarked in 

 passing, that the relatively low consolidating temperature of 

 eutectic mixtures compared with those of the individual con- 

 stituents is in keeping with the fact that the ore was the last 

 portion of the rock to consolidate. 



It appears clear to the writer that the feldspar was the first 

 mineral to separate from the magma and that the olivine and 

 the ore then separated in some manner from each other and 

 crystallized. The crystallization of so large a propor- 

 tion of the magma (ore plus olivine nearly 85 per cent) 

 must have given rise to a considerable evolution of heat, in 



