Geology and Mineralogy. 241 



*gan, the water falling in the north and rising in the south, at 

 Chicago some nine or ten inches. The ultimate result would be 

 the discharge of the lakes through the Illinois river, the channel 

 of the Pleistocene glacial lake being occupied anew. The high- 

 water lake discharge may begin in 500 to 600 years, for mean 

 lake stage it will begin in 1,000 and after 1,500 it will go on 

 uninterruptedly. In about 2,000 years the Illinois river and 

 Niagara will carry equal portions of the surplus water of the 

 Oreat Lakes, and in 3,500 there will be no Niagara. The author 

 adds : " The most numerous economic bearings of this geographic 

 change pertain to engineering works, especially for the preserva- 

 tion of harbors and regulation of water levels. But the modifica- 

 tions thus produced are so slow as compared to the growing 

 demands of commerce for depth of water that they may have 

 small importance. It is a matter of greater moment that cities 

 and towns built on lowlands about Lakes Ontario, Erie, Michigan, 

 and Superior will sooner or later feel the encroachment of the 

 advancing water, and it is peculiarly unfortunate that Chicago, 

 the largest city on the lakes, stands on a sinking plain that is now 

 but little above the high-water level of Lake Michigan." 



The paper closes with a statement of certain definite plans 

 inade for precise measurements at selected stations, for example, 

 at Chicago, Port Huron, Parry Sound, and Mackinaw, which if 

 they can be carried out systematically, must in time lead to a 

 very definite solution of the problem in hand. 



2. On the Petrology of Rockall Island. — Professor J. W. Judd 

 has recently described a remarkable rock under the name rockallite 

 from the isolated point which rises from a shoal above the waves 

 of the North Atlantic 240 miles west of Ireland. It appears to 

 be a remnant of an intrusive sheet resting on sediments. The 

 tiny islet has long been known under the name of Rockall. Prof. 

 Judd's paper forms one of a series of memoirs descriptive of 

 this remarkable island.* As this rock is of very interesting char- 

 acter a brief description will be of interest to petrographers. 



It is of granitic character but with an approach to a porphyritic 

 structure in places owing to the idiomorphic outlines of the lath- 

 shaped feldspars, which allies it to the granite porphyries. In 

 composition it is very simple, being made up of asgirite, albite and 

 quartz in the following proportions : aegirite 39, quartz 38, 

 albite 23. 



The segirite is found in larger crystals and in fine needles, and 

 shows at times brown acmite zones. The albite is twinned accord- 

 ing to the albite and Carlsbad laws and its phenocrystic sections 

 contain aegirite. The quartz is the last mineral to form and fills 

 the interspaces. A little apatite and soda amphibole are present. 

 The specific gravity varies from 2*9-2 -7. In the more porphyritic 



* Notes on Rockall Island and Bank with an account of the Petrology of Rock- 

 all, and of its winds, currents, etc. : with reports on the Ornithology, the Inverte- 

 brate Fauna of the Bank and on its previous history. Transactions of the Royal 

 Irish Academy, vol. xxxi, Part III, pp. 39-98, plates ix to xiv. 



