16 Johnson and Warren — Geology of Rhode Island. 



ture extends everywhere through the ore except over occasional 

 small areas which have a smooth surface and are identical 

 with the bands in appearance and presumably in composition. 

 The lamellae are frequently observed to cross at angles of nearly 

 or exactly 60 degrees; again they form a nearly rectangular 

 grating. This structure taken in connection with the well known 

 tendency of ilmenite to develop a reticulate structure parallel 

 to the rhombohedral planes, suggests at once that the lamellae 

 are ilmenite enclosing the more easily dissolved magnetite. 

 The change in direction of the bands in different parts of the 

 ore matrix is the only indication of the existence of in- 

 dividual grains in the ore. 



The ratios derived from the rock analysis show conclusively 

 that the two molecules, R ZI Ti0 3 and R, n E, m 2 4 , are present, 

 and in about equal proportions. The magnetic susceptibility 

 of particles of the ore, free from olivine and feldspar, was 

 found to be intermediate between that of pure magnetite and 

 ilmenite. It appears, therefore, that the ore matrix consists 

 essentially of an intimate intergrowth of magnetite and 

 ilmenite with an occasional grain of what is probably pure 

 ilmenite. The parallel intergrowth of members of the spinel 

 family with ilmenite has long been familiar to mineralogists 

 and a similar intergrowth has been suggested by a number of 

 petrographers to explain the composition of the " titaniferous 

 magnetites" of many rocks. Quite recently Hussak* has 

 shown by the study of etched surfaces that intergrowths of 

 ilmenite and spinel minerals, particularly magnetite, are very 

 general in the titaniferous ores of Brazil. He also gives a full 

 bibliography of the literature on the subject. It would be 

 extremely interesting to extend the study of titaniferous mag- 

 netites still further and ascertain to what extent, if at all, an 

 intergrowth like the one described in the present instance 

 exists in other occurrences. There is still, in spite of the con- 

 siderable amount of investigation which has been carried on 

 upon " titaniferous-magnetites," much uncertainty regarding 

 the exact relations of these two mineral molecules when 

 occurring together. 



Spinel. — Examined in thin section the edges of the ore show a 

 fine granularity under high powers. Numerous small crystals of 

 a dark green, feebly transparent, isotropic mineral of an angular 

 or sub-angular habit are enclosed at random in the ore. They 

 are never found outside of it. A few of the grains have distinct 

 crystallographic outlines suggestive of isometric crystal sections, 

 and many are crossed by sharp, straight (cleavage ?) cracks. The 

 ciwstals vary in size from exceedingly minute individuals up 

 to those measuring - 45 mra long by 0*12 mm wide. They occur 



* Jahrb. f. Min., etc., Bd. I, Heft II, 1904, pp. 94-113. 



