Olivine. Fayalite. Bowlingite.] 

 The iron ores. 



MINERALOGY AND PETROLOGY. 963 



Olivine. Normally olivine seems to have existed in all the original basic igneous 

 rocks, but it was scattered in small grains orcrystals without existing in large quan- 

 tities. It is also in the secondary basic igneous rocks of the Keweenawan, but in 

 some large areas it is quite sparse in the gabbro, as in the anorthosytes. It is some- 

 times found in great abundance in the muscovadyte, where it is surrounded with 

 unusual mineral associates. 



In the original basic rocks of the Keewatin, supposed to be the representative 

 of the first crust of the earth, and hence the oldest rocks in the state, olivine cannot 

 now be recognized as such, but has been destroyed by the vicissitudes of the long 

 history which it has experienced, some of its serpentinous alteration products only 

 being left to bear testimony to its earlier existence (Nos. 349, 2158). 



In the Keweenawan, olivine is not always one of the earliest of the minerals. 







This occurs in some of the coarse gabbros, or coarse diabases (Nos. 258, 512, 603, 

 787, 819, 1275, 1828, 1829, 429E) quite frequently, and occasionally it has been noted 

 in narrow dikes (No. 757). This late generation of olivine, compared with that of 

 the feldspathic individuals of the basic Keweenawan, therefore, is not due, apparently, 

 to any batholitic conditions of consolidation, but to chemical conditions inherent in 

 the magma. 



Fayalite. That form of olivine which is characterized optically by its negative 

 bisectrix and a distinct cleavage (010) has been recognized in several instances in 

 the muscovadytes (Nos. 1041, 1336, 1343, 1365, 1829, 2058, 2199). This seems to be 

 most common where the rock contains also considerable magnetite. 



Bowlingite. Olivine has been seen transformed not only into an indefinite more 

 or less fibrous substance which usually is denominated serpentine, and sometimes 

 into a more definitely characterized mineral (antigorite), but also to the mineral 

 named by Hannay* bowlingite (Nos. 193, 560, 703). Sometimes a grain of olivine is 

 wholly transformed to bowlingite, which takes the form and place of the original, 

 and sometimes a rim of bowlingite surrounds feldspar grains which are also embraced 

 in magnetite, separating the feldspar entirely from contact with the magnetite. 

 This occurs in the cumberlandyte of Mayhew lake (No. 703). 



The iron ores Ilmenite, magnetite. These are mentioned in conjunction for' the 

 reason that they seem to be equally old and intimately associated, and have not usually 

 been separately determined in the course of the microscopical examinations. Whether 

 the titaniferous element seen in the secondary minerals, rutile, leucoxene, etc., is 

 dependent on ilmenite or a titaniferous magnetite, or whether the iron ore that 

 occurs sometimes in large masses in the gabbro and in the muscovadyte is ilmenite 

 or titaniferous magnetite, in the majority of cases cannot be stated. On the other 



'Mincralogical Magazine, vol. i, p. 164, 1877. 



