Intelligence and Miscellaneous Articles. 243 



tensive veins and lenticular masses of quartz, and are traversed by 

 basalts. The Silurian strata, resting unconfo-rmably on the gneiss, 

 have been invaded byjigneous matter (which is never granitic), and, 

 though generally horizontal, arc frequently nexuous, and in some 

 places greatly faulted, to the extent of even 1000 feet, together 

 with the gneissic rocks beneath. These latter have been deeply 

 eroded by the rivers, frequently to the depth of 500-1000 feet, and 

 even of 3000 feet in some valleys ; and in the alluvia of these 

 valleys the gold occurs. The valleys have sometimes evidently com- 

 menced in great displacements, forming " valleys of elevation," on 

 which the denuding agency has been operating ever since. 



In certain mountains in the basin of the St. John's River, 

 Natal, dioritic rock traverses the secondary strata ; and along the 

 line of contact it contains copper-ores with 100 grains of gold to 

 the ton. 



Mr. David Forbes was glad to find that Dr. Sutherland corrobo- 

 rated his views as to the occurrence of gold in two ways : — ■ 



1. In auriferous granite, as in Wicklow and elsewhere. 



2. In eruptive dioritc, a basic rock without free quartz, and cer- 

 tainty of postoolitic date, almost always accompanied by copper 

 veins. Host Californian alluvial deposits of gold were derived from 

 this class of rocks. 



In constructing some of the railways of South America the granite 

 was found to be so soft, from decomposition, that it could be cut with 

 the pick and spade ; and this softened granite, when washed, pro- 

 duced gold. 



Prof. T. Rupert Jokes considered that, by means of Dr. Suther- 

 land's communication, the Laurentian and Silurian rocks were now, 

 for the first time, to be recognized as existing beneath the Dicy- 

 nodon^-YOcks of the Natal ridge. 



XXIX. Intelligence and Miscellaneous Articles. 



NOTE ON ELECTROLYTIC POLARIZATION. BY PROFESSOR TAIT. 



T HAD just obtained one of Sir W. Thomson's most recent forms 

 -*■ of quadrant electrometer, and it occurred to me that this must be 

 the proper instrument for determining polarization, as its indications 

 are not affected by electric resistance, and give directly (that is, 

 without assuming the truth of Ohm's law for reverse electromotive 

 forces, and the consequent necessary determinations of resistance) 

 the quantities required. The method employed by Wheatstone, 

 Poggendorff, Buff, and others assumes that the whole electromotive 

 force in the circuit is the algebraic sum of those of the decomposing 

 battery and of the electrodes — an assumption whose truth some may 

 consider to require proof, and which it is certainly useful to verify 

 by an independent process. Again, after the decomposing action 

 has ceased, the resistance of the films (of gas or oxide) which are 

 deposited on the electrodes may change in value. That neither of 



