346 DR. r. DixET OjS" the [vol. Ixxviii, 



occupied by g-ranules of felspar. The hypersthene has largely 

 recry stall] zed in granular form ; it contains much magnetite, of 

 which some at least has been thrown down as a result of thermal 

 metamorphism. The olivine has recry stallized locally, but only 

 to a slight extent. Along its contact with the hypersthene the 

 olivine has been converted into serpentine ; this serpentine appears 

 black in the photograph. Magnetite has been thro^aa do-wn along 

 cracks in the olivine. Ordinary light. X 28. (See p. 339.) 

 Fig. 4. Corroded augite - xenocry st in beerbachite [C 101], 

 Wilber force. — The minerals seen here are felsj^ar, pyroxene, 

 and magnetite. The augite-xenocryst, derived from the norite, 

 underwent much corrosion ; in the course of this corrosion the 

 crystal was rendered almost opaque by the deposition of much 

 iron-ore in the form of s chiller -inclusions. Then, as the en- 

 closing magma began to crystallize, a clear outgro^vth formed 

 around the xenocryst ; this outgrowth is in optical continuity with 

 the augite of the original crystal. Ordinary light. X 28. (See 

 p. 339.) 



Figs. 5 & 6 illustrate the alteration of norite by a relatively- acid 

 magma : namely, norite-aplite. 



5. Alteration of augite by norite-aplite [CllGci], near 

 York. In the centre of the field of view is a crystal of augite 

 which has been almost completely replaced by hornblende in the 

 form of ragged green plates ; remnants of the original crystal may 

 still be seen as small light- coloured shreds. Numerous grains of 

 iron-ore are scattered through the crystal ; they were thrown down 

 as a result of the alteration. In the lower part of the figure the 

 pseudomorph is seen to be in communication with a small vein of 

 norite-aplite that was chiefly responsible for the alteration. The 

 felspar shown in the figure has also been considerably modified ; 

 it has on the whole become less turbid, although numerous inclu- 

 sions have developed in it. In the upper left-hand quadrant is a 

 patch of turbid felspar which shows only slight traces of alteration. 

 Numerous shreds and small spongy masses of hornblende are 

 enclosed in the felspar ; at least some of these represent augite 

 which originally had been intergrown with the felspar in graphic 

 form. Ordinary light. X 28. (See p. 341.) 

 < 6. Alteration of olivine in norite [C 172d], near York. 

 An altered crystal of olivine is seen in the centre of the field 

 of view ; part of another similar crystal is seen in the upper left- 

 hand quadrant. The olivine has been rendered almost opaque by 

 the deposition of much magnetite in the form of dust and small 

 grains. Around the margins the olivine has been largely replaced 

 by flakes of brown biotite, as may be seen in the upper right-hand 

 quadrant and along the lower margin of the crystal of olivine. 

 An interesting feature of this contact-alteration of the norite is the 

 abundant development of micrographic felspar and augite, which 

 in the present instance almost completely su.rrounds the altered 

 olivine, and spreads more or less fan-wise into the adjacent felspar. 

 The intergrowth takes the form of small curved rods and blebs of 

 . augite embedded in felspar. Ordinary light. X 28. (See p. 341.) 



