1886. ] Geology and Faieontology. 57 
balance continental masses having an average specific gravity of 
2.5. Ifthe specific gravity does not increase downward as rap- 
. idly as the rate assumed, as we may infer from Mr. Pierce’s table 
» 
(U. S. Coast and Geodetic Survey, 1879, p. 200), then a still 
greater depth would be required to secure equilibrium. From 
these considerations it would appear that the superficial continen- 
tal strata must have an average specific gravity much below 2.5 
and, in order that this may be so, that much material must have 
been removed from within the mass.—/. H. King, River Falls, 
Wis., May 12, #885. 
GEOLOGICAL Survey oF Betcium—In 1878 a commission was 
appointed to undertake a more exhaustive invéstigation of the 
geology of Belgium than that embodied in the map of Dumont. 
The topographical map serving as a basis consists of 369 sheets. 
Each important group of formations is entrusted to one or more 
specialists, who are each furnished with two assistants, and trace 
the system completely across the country. Every actual outcrop 
of rock is marked on the map, and where the rock is fossiliferous 
the fossils are noted. Special attention is given to soils and su 
soils, and care taken to express on the map the agricultural char- 
acter of the ground. It is believed that one-third of the entire 
work of the survey is now completed. By a novel system of 
broad washes of subdued tints, M. Dupont, the head of the sur- 
vey and Director of the Royal Museum at Brussels, contrives to 
show the surface deposits, as well as the formations below, which 
are shown in deeper tints; while shaded lines of the proper color 
mark the margins of the stage. Professor Archibald Geikie ex- 
presses in Nature his conviction of the success of the new system 
of cartography. 
THE BED or THE Ocean.— The Tuesday evening discourse during 
the late meeting of the British Association was delivered by Mr. J. 
Murray, F.R.S., of the Challenger expedition, who took for his 
subject the “ Bed of the ocean, and some results of the expedi- 
tion.” In commencing his lecture, Mr. Murray traced the devel- 
opment of geographical knowledge from the crude conception of 
the ancients down to the ‘extended knowledge of the nineteenth 
century. It was not easy, he said, to estimate the relative impor- 
tance of the events of one’s own time, yet, in all probability, the 
historians of the reign of Victoria would point to the recent dis- 
coveries in the great oceans as the most important events of the 
century with respect to the acquisition of natural knowledge—as 
ong the most brilliant conquests of man in his struggle with 
- Rature; and doubtless they would be able to trace the effects of 
these discoveries on the literature and on the philosophic concep- _ 
tions of our age. The last of the great outlines showing the sur- . 
face features of our globe had been boldly sketched ; the founda- a 
tions of a more complete and scientific physiography of theearth’s 
