December 7, 1893] 



NATURE 



131 



action of heat upon the Santa Lucia mountain. According to La 

 Nature, the observatory has recently been removed to a plain at the 

 south of the city, and Dr. Obrecht, the director, has investigated 

 the movements. It appears from his observations that from 

 noon until nine o'clock in the evening, the ground to the north- 

 east is raised, and then gradually descends until seven o'clock 

 on the following morning. These curious variations sometimes 

 attain an amplitude of 3" or 4". There is also evidence that 

 from July to September the ground to the north-east is continu- 

 ously raised, while from September to November, the part to 

 the east of the observatory is continuously elevated. The total 

 amplitude of elevation is said to be about 35". 



Mr. a, Sigson, a professional photographer at Rybinsk, 

 contributes an account of his method of obtaining photographs 

 of snowflakes to the Journal of the Russian Physico-Cheniical 

 Society. He used a Zeiss microscope provided with an aplanatic 

 lens and a long focus camera. This was placed near an attic 

 window at a strong inclination to the horizon. The flakes were 

 received on some rough cloth and transferred to a small net of 

 cocoon fibres stuck on a card perforated in the middle. This 

 card was placed on the stage of the microscope, and the illumin- 

 ation was so arranged that half the field was uniformly illumin- 

 ated, and the other half shaded off. For an enlargement of 

 fifteen times the exposure lasted two to five seconds, with plates 

 supplied by M. Lumiere. To avoid the melting of the flakes 

 by the breath of the operator, the latter is obliged to breathe 

 through a pipe bent backwards during the adjustment of the 

 apparatus. 



In Bulletin No. 8 of the Geological and Natural History 

 Survey of Minnesota, Dr. Andrew C. Lawson publishes two 

 papers of great importance for the systematic grouping of 

 volcanic rocks in North America. The first paper is on the 

 " Anorthosytes of the Minnesota Coast of Lake Superior," and 

 is prefaced by a long note, written by Prof. Winchell, on "The 

 Norian of the North-West." In this note Prof. Winchell gives 

 up many of his previously-formed ideas regarding the Minnesota 

 rocks, in favour of the conclusions now obtained by Dr. Lawson. 

 There occurs on the Minnesota coasts a rock almost wholly com- 

 posed of a plagioclase felspar which had been included by Profs. 

 Winchell and Irving in the Keweenian or Cupriferous series of 

 volcanic lavas and sheets. For this rock, Dr. Lawson accepts the 

 name of " Anorthosyte," given by Prof. Adams to similar rocks 

 in the Norian series of Quebec ; and he proves conclusively 

 that it is a Plutonic formation, solidified under deep-seated con- 

 ditions, and exposed later during the long period of pre-Palse- 

 ozoic erosion. On its eroded surface the volcanic lavas of the 

 Keweenian series were poured out, no rocks belonging to the 

 Animikie series being present in this area. The thickness of the 

 Keweenian series had been estimated by Prof. Irving at 20,000 

 feet. Dr. Lawson is of opinion that the series is comparatively 

 thin, ranging from zero to a maximum of a few hundred feet. 

 Special interest attaches to the hummocky — rocJies moutonees — 

 aspect of the old surfaces of the Anorthosyte 'rock at Beaver 

 Bay, Carlton Peak, &c., as this is such a marked feature of the 

 ancient erosion planes of Archsean rocks in North America. 

 Dr. Lawson compares the Anorthosytes of Minnesota with the 

 Norian series of irruptive plagioclase rocks invading Archaean 

 gneisses in Quebec, but until there is sufficient evidence in 

 favour of this correlation, he suggests that a local name of 

 " Carltonian " be given to the Minnesota Anorthosytes, 



The second paper in the same Bulletin, by Dr. Lawson, is 

 entitled "The Laccolitic Sills of the North-West Coast of Lake 

 Superior." Extensive trap-sheets are in this region associated 

 with the Animikie and Nipigon groups of sedimentary rocks, and 

 have up to this time been described as contemporaneous flows. 

 Mr, Ingall had observed the intrusive nature of some of these 



NO. 1258, VOL. 49] 



so-called flows, but drew no farther conclusions. Dr. Lawsoa 

 now advances the view that "there are no contemporaneous 

 volcanic rocks in the Animikie group, and that the trap-sheets 

 are all intrusive in their origin, and are of the nature of 

 laccolitic sills. " He supports this view by weighty evidence, 

 such as the simplicity of the trap-sheets, their regularity and' 

 persistence over wide areas, the passage of thick sheets from 

 the Animikie series into the higher horizons of Keweenian 

 strata, the absence of pyroclastic rocks, the alteration of the 

 rocks above and below the intruded sheets, and the direct con- 

 tinuity of the "trap-sheets" with dykes of the same intrusive 

 rock. The "trap-sheets" occur as laccolitic sills both in the 

 Animikie and Keweenian series, and are therefore later than 

 these. Dr. Lawson thinks they may belong to the great series 

 of trap-rocks intruded in the Silurian rocks of Quebec, but calls 

 them for the present " Logan Sills," in honour of the late Sir 

 William E. Logan. 



It is well known that electric currents may be produced by^ 

 heating a single metal, if there be any variation in temper, or if 

 the distribution of heat be very irregular, and the changes of 

 temperature abrupt. Mr. W. H. Steele has made some experi- 

 ments on these effects, in the Physical Laboratory of Melbourne 

 University {Science, No. 562). A sensitive galvonometer put 

 in circuit with a piece of iron wire showed a current when the 

 wire was simply warmed with the fingers. This was the only 

 metal which gave a carrent when at a temperature below 

 100° C. Altogether twelve different metals and four alloys 

 were examined, and the effect noticed in each of them. In order 

 to raise the wires to a high temperature without fusing them, 

 they were passed through clay tubes (stems of tobacco-pipes), 

 and, in the case of metals having low melting-points, the tube 

 was completely filled with the metal. The highest electro- 

 motive force obtained from iron w.is 0-002 volt ; 0-3 volt was 

 observed with six different metals— lead, copper, gold, tin, zinc, 

 and antimony ; while with others, e.g. silver and aluminium, 

 the effect was exceedingly small. In the case of lead, the effect 

 showed no sign of ceasing after the metal had been heated 

 for half a day. Gold gave the highest effect, as much as 

 half a volt being observed. Mr. Steele remarks that these 

 phenomena are generally quite sufficient to mask the ordi- 

 nary thermo-electric effect at a red heat, and that thermo- 

 electric tables are consequently not trustworthy for high 

 temperatures. 



The current number of the Comptes Rendus contains a note, by 

 M. Ch. Andre, on the variation of the electric state of the high 

 regions of the atmosphere in fine weather. Daring a previous 

 attempt to investigate this point, the author unfortunately met 

 with an accident which has prevented him personally making 

 any more observations ; the measurements contained in this 

 note have, however, been made under his direction. At oppo- 

 site corners of the car of the balloon were fixed two cylindrical 

 reservoirs, filled with distilled water, and insulated on plates of 

 sulphur. To the base of each of these vessels an india-rubber 

 tube, about 20 metres long, was attached, each tube having a 

 small jet at its end. When the balloon had come to rest at any 

 desired height, the difference of potential existing between two 

 points, at a known vertical distance, was determined by means 

 of an electrometer (Exner's pattern) connected metallically 

 with the water reservoir. This difference of potential, 

 the distance being kept constant, gave a measure of the 

 strength of the electrical field. As a result of two series of ob- 

 servations, the author considers that in fine weather the strength 

 of the electrical field does not increase with the altitude, but is 

 the same at a given instant at any point along the same 

 vertical. 



