Vol. 50.] ANNIVERSARY ADDRESS OF THE PRESIDENT. I I 5 



fragments. Since there is no evidence of any portion of the plutonic 

 rock having been affected by earth-movements, in this case the 

 Authors are disposed to attribute foliation to movements anterior 

 to final consolidation. 



The Authors conclude that in this area is the record of a series of 

 events connected with the consolidation of a vast subterranean 

 reservoir of molten rock, and the nature of the resulting complex 

 they proceed to describe. The first rocks to be formed were perido- 

 tites ; then followed diorite, tonalite, granite, and eurite (felsite) 

 in order of increasing acidity. The most acid rock known occurs in 

 narrow veins in the granite and tonalite, and is almost entirely 

 devoid of ferro-magnesian constituents. Probably Mr. Barrow 

 would suggest that this last rock represented the remains of the 

 mother-liquor in which the granite, etc., had consolidated. The 

 Authors further describe the physical features of many of the rocks, 

 giving at the same time a detailed account of the constituent minerals. 

 A comparison of the general distribution of these shows that, in 

 this area (where it so happens that ordinary gabbros are absent), 

 in proportion as the olivine dies out pyroxenes increase in importance, 

 and these, in turn, are replaced by hornblende and biotite. Next 

 the hornblende decreases relatively to the biotite, and finally, in the 

 eurite-veins, the ferro-magnesian silicates have entirely disappeared. 

 Turning to the quartzo-felspathic constituents and considering their 

 distribution in the same way, it is observed that plagioclase first 

 makes its appearance, then follows orthoclase and lastly microcline ; 

 quartz comes in with the orthoclase. It is instructive, they say, to 

 note that when the minerals pyroxene, hornblende, biotite, plagio- 

 clase, microcline, and quartz make their first appearance, they play 

 the role of groundmass ; in other words, they are allotriomorphic or 

 ophitic with respect to the other constituents. It is only when a 

 mineral has established itself as an important constituent that it 

 begins to show traces of idiomorphism. Thus in the order of for- 

 mation of minerals a certain amount of overlapping takes place, and 

 this overlapping is stated to reach its maximum in the plutonic 

 rocks. The following order of occurrence, deduced from these 

 investigations, accords on the whole with previous experience : — 

 Iron-ores, olivine, pyroxene, hornblende, biotite, plagioclase, ortho- 

 clase, microcline, and quartz. Accessory minerals are singularly 

 absent in the ultra-basic rocks. 



Much attention has been paid by the Authors to the chemical 

 composition of these rocks, and very interesting generalizations thus 

 deduced. In accordance with precedent, a kind of chemico-minera- 



