Geology and Mineralogy. 153 



stitutes in the same voltameter some very fine electrodes there is 

 an abundant disengagement of gases. The authors give various 

 examples to show that electrolysis by alternating currents is dif- 

 ferent from that by continuous currents. By a suitable choice of 

 electrolyte, an alternating current can be measured by a volta- 

 meter. — Comptes Rendus, p. 1719, June 18, 1888. j. t. 



6. Electro- chemical Radiophone. — Chaperox and Mercadier 

 employ a galvanic cell composed of one plate of silver coated by 

 electrolysis of sulphide of sodium with a very thin layer of sul- 

 phide of silver, and another plate of silver. Both plates are 

 placed in a liquid conductor, water acidulated with sulphuric acid, 

 for instance. The cell possesses a very feeble electromotive force. 

 It polarizes very rapidly and gives rise to variable instantaneous 

 currents under the action of very feeble light. By means of a ra- 

 diophonic wheel an intermittent beam of light corresponding to a 

 tone of 1000 vibrations was allowed to fall on the cell, and the 

 note was heard in a telephone enclosed in the circuit. This note 

 was due to an electrochemical effect, the duration of which was 

 less than -giroo °f a second. — Comptes Rendus, No. 23, p. 1595. 

 June 4, 1888. j. t. 



II. Geology and Minekalogy. 



1. On the Black Hills of Dakota ; by Prof. W. l O. Crosby,— 

 A paper in the Proceedings of the Boston Society of Natural 

 History for March 7, 1888 (p. 488), describes the rocks of the 

 Black Hills, and shows that the great granite masses, which occur 

 in the western portion of the Archaean area, consist really of vein 

 granite. This is shown by their position and coarseness of 

 texture. He makes also the important observation that the high 

 conical masses of rhyolyte and trachyte of the northern end of 

 the Black Hills, described by Prof. Henry Newton, are true lacco- 

 liths. Speaking of the Paleozoic rocks he remarks on the perfect 

 conformability of the Carboniferous beds on the Cambrian with 

 which they are in contact, the intervening Silurian and Devonian 

 being absent. The unconformability of the Cambrian on the 

 Archaean is to be seen at all contacts. 



2. Annual Report of the Geological Survey of Pennsylvania 

 for 18S6. — Parts II and III of this report have been issued. Part 

 II treats of the Oil and Gas Region, and contains the detailed 

 report of Mr. J. F. Carll, covering over 200 pages and also a paper 

 on the Chemical Composition of Natural Gas by Prof. F. C. 

 Phillips. The gas of Fredonia, N. Y., was found to contain 90*05 

 p. c. of hydrocarbons of the paraffin series (in which methane 

 predominates), 9*54 nitrogen and 0*41 carbon dioxide. This is about 

 the average of the results obtained. But the Murrysville gas, a 

 very productive well, afforded 97*70 of paraffins and only 2*02 

 nitrogen, and 0*28 C0 2 ; while the Houston gas, from a well two 

 miles south of Canonsburg, Pa., afforded 84 - :26 paraffins, 15*30 of 

 nitrogen and 0*44 C0 2 . 



