310 ANNUAL OF SCIENTIFIC DISCOVERY. 



to have escaped both Dr. Livingstone and Sir Roderick Murchison that the 

 wide north and south cleft through the plateau, at the bottom of which the 

 Congo meanders, must have been occasioned by just such a broad anticlinal 

 wave in the rocks as that which geologists in America call the Cincinnati 

 axis, separating the eastern and western coal-fields, although its nearest 

 likeness is to the north and south valleys of the Jordan and the Nile. 



CONDUCTING TOWER OF ROCKS ALTITUDE OF MOUNTAINS NOT 

 INVARIABLE. BY CHARLES MACLAREN. 



Mr. Hopkins, of Cambridge, has made some rather interesting experiments 

 on the conductivity or conducting power of different substances for heat, of 

 which an account was laid before the Royal Society of London, in June last. 

 Without attempting to describe his processes, we give his more important 

 results, and in decimals, the conductivity of " igneous rock " (trap or granite, 

 we presume), saturated with moisture, being taken as unity. 



Chalk, in the state of dry powder -056 



Clay, " " -070 



Sand, " " -150 



Sand and clay, " -110 



The conductivity of the following rocks is given in two states dry, and 

 saturated with water : 



Dry. Saturated. 



Chalk in block -17 -30 



Oolite rock -30 -40 



Hard compact limestones '50 '55 



Siliceous new red sandstone -25 '60 



Freestone -33 -45 



Hard compact sandstone (millstone grit) -51 "76 



Hard compact old sedimentary -50 '61 



Igneous rocks '53 1.00 



The effect of pressure on the conducting power of substances was also 

 tried, and proved to be almost nothing. A pressure of 7500 Ibs. on a square 

 inch of bees-wax, spermaceti, and chalk, had no appreciable effect. Uncom- 

 pressed clay, which had a conducting power of '26, had the same raised to 

 33 by a pressure of 7500 Ibs. 



Sandstone, with conducting power of '5, divided into strata each one foot 

 thick, when compared with a similar mass in one block, had its conducting 

 power diminished ^ l gth. When the strata were only six inches thick, the 

 diminution was -j-^th. The effect of discontinuity of substance is, therefore, 

 small. Saturation with moisture, on the other hand, produces generally 

 a great effect, as will be seen on comparing the dry and saturated blocks of 

 chalk, the dry and saturated new red sandstone, and again the dry and sat- 

 urated " igneous rocks." 



These facts have a certain bearing on a geological question namely, the 

 transmission of heat from the interior of the earth to the crust. The oolite, 

 for instance, conducts heat much better than the chalk, the sandstone better 

 than the oolite, the igneous rock better than the sandstone, and in all cases 

 the rock charged with moisture better than the dry rock. But Mr. Hopkins 

 would have added to the value of his paper, if he had ascertained by experi- 



