OLDER PERIDOTITES OE SCOTLAND. 417 



SchiUerization might prove useful, but he questioned whether some 

 of the results attributed to it might not be due to weathering. 



Mr. Baijerman felt the same difficulty that had been alluded to 

 by the previous speaker in entering upon the discussion of this 

 paper. He remarked that the twinning of triclinic felspars was 

 not an essential peculiarity, as he had seen specimens in which it 

 was absent ; but he considered the origination of twinning from 

 pressure unproved, as there was no difference of cohesion in diffe- 

 rent directions sufficient to produce twinning. Twinning is espe- 

 cially well seen in albite crystallized freely in druses in mineral 

 veins. Pseudostructures of lower symmetry in boracite &c. are 

 due to change of structure produced in cooling, and quite distinct 

 from twinning in felspars. 



Rev. E. Hill said that all banding that he had observed in igneous 

 rocks, glass, &c. varied gradually from point to point when due to 

 pressure, and thus differed from that described by the Author. 



Mr. HuDLESTON remarked that magnesian rocks were very 

 mysterious. Their peculiarities are perhaps due to chemical com- 

 position. Gabbros are always found associated with serpentine and 

 olivine rocks. He inquired whether SchiUerization might not be 

 due partly to change of structure arising from something inherent. 

 As a case of change resulting from original peculiarity of compo- 

 sition, he quoted the analysis of a rock lately described by Mr. 

 Teall, which showed an amount of magnesia unusual for a non- 

 olivine dolerite. There was no mineral in that rock to absorb so 

 much magnesia except the augite, which must have possessed an 

 exceptional composition, and consequently an inherent tendency to 

 change : such inherent tendency might help to explain the phe- 

 nomena of SchiUerization. He further commented on the alterations 

 stated to have taken place in depth; for some Schillerized rocks, e.g. 

 bastite, are hydrated forms. 



Prof. Hughes asked how far the relations of the rocks in 

 question to one another and the conditions of depth &c. could be 

 considered well established. In the example exhibited he thought 

 the vein called gabbro was due to alteration along a joint, of which 

 he saw traces on the back of the specimen in the divisional plane 

 which ran almost through the middle of the vein. It looked like 

 a vein along a faulted joint, and might be of any age later than the 

 olivine rock and subsequent to the jointing and faulting, and there- 

 fore not belonging to deep-seated conditions. 



The President congratulated the Author on having, as he said^ 

 driven another nail into the coffin of the classification of igneous 

 rocks by their geological age. He had always believed that the 

 altered peridotites of the Apennines were of Tertiary age. In 

 all cases that he had seen, the gabbro and peridotite were quite 

 independent rocks ; but he had seen picrite pass into diorite. He 

 himself had always found gabbro the newer rock, evidently deep- 

 seated ; for the crystallization was coarse even in small veins. He 

 also doubted whether plagioclastic twinning was due to pressure ; 

 for it is found in rocks not much pressed, such as lava-streams. 



