254 



NA TURE 



[January i6, 1896 



coveries in many parts of the world. A rich marine Triassic 

 fauna is now known extending from Spain to Japan and Cali- 

 fornia, and from Spitzbergen to New Zealand. Yet among the 

 thousands of these fossils gathered together in Vienna from all 

 parts, there is not a single marine fossil from the regions boider- 

 ing the Atlantic or Indian Oceans. The conclusion is obvious, 

 that the regions of these modern oceans were not covered by sea 

 in Triassic times. On the other hand, all the districts bordering 

 the Pacific and Mediterranean yield the marine forms, as does a 

 great stretch of land extending from the Mediterranean to the 

 Pacific through Central Asia, and another extending from the 

 Pacific through Eastern Siberia to the Arctic Ocean. Thus the 

 Pacific Ocean was the main ocean in Triassic times, and stretched 

 out two arms across the continental region — the one called the 

 Tethyan ocean, of which the Mediterranean is the last remnant, 

 the other the Arctic branch. This distribution of the Triassic 

 seas strikingly agrees with that of the structural features of 

 modern coast-lines indicated by Neumayr : the oceans bordered 

 by lands with marine Trias are the oceans of the Pacific type, 

 of which the coasts are determined by the convex margins of 

 earth-folds ; while the oceans of Atlantic type, of which the 

 margins cut across the mountain-folds, are those around which 

 only the fresh-water Triassic strata are found. Thus is con- 

 firmed the opinion that the latter oceans are of comparatively 

 recent origin, and have been produced by a process of wholesale 

 depression, which has cut out the three great triangular up- 

 standing masses (or horsts) of Greenland, Africa, and India, 

 which form so striking a feature on the surface of our planet. 



The Geological Survey has generally been successful with its 

 index maps, and the beautiful hand-coloured index of Wales, 

 on the scale of four miles to an inch, has a great reputation for 

 correctness of topography, clearness and accuracy of colouring, 

 and beauty of appearance. After the lapse of a long period, 

 the Survey, about two years ago, began the issue of an index 

 map of England and Wales on the same scale, and of this 

 several sheets have been already issued. These sheets were also 

 coloured by hand ; but on account of their complicated structure 

 they were necessarily very expensive to produce, and hence, 

 although sold with the barest possible margin of profit, or, 

 indeed, with none at all, they could never become very widely 

 used in consequence of their high price. Now, however, an 

 important experiment has been tried, the issue of Sheet 12 

 printed in colours. This map includes the London Basin and 

 the greater part of the Weald, and it is identical with the hand- 

 coloured sheet ; but whereas the price of the latter was loj-. 61a?'., 

 that of the former is 2s. 6d. Printing the map in colours brings 

 with it practically no disadvantage. The topography is clear 

 and correct, the latest railways being inserted ; the colours are 

 transparent, clean, harmonious, and well-defined ; each colour 

 is lettered at all critical points ; the registration is wonderfully 

 exact, although such minute delineation is a very severe test of the 

 workmanship ; and the detail is quite as full as on the hand- 

 coloured map, which, it will be remembered, contained many 

 revisions not as yet shown on the original i-inch sheets. On 

 the other hand, printing in colours has two inestimable advan- 

 tages, which those familiar with the hand-coloured maps will 

 realise to the full ; the inevitable omissions of the colourist will 

 disappear, and, as the colour-proofs come to hand, there is every 

 encouragement for the makers of the map to exercise the 

 minutest care in delineations of details, which, once inserted, 

 cannot again drop out by accidental omissions of the draughts- 

 man, engraver, or colourist. It is to be hoped that this is only 

 the beginning of good things, and that the Survey will persevere 

 until it has the rest of the index similarly printed, thus providing 

 an unequalled and authoritative map of the whole country. 

 If this is successful, we cannot stop here, but must look forward 

 NO. 1368, VOL. 53] 



to having reproduced by colour-printing at least such of the 

 I -inch sheets as are sufficiently used to justify the expenditure. 

 Indeed, it would doubtless be wisest, and really cheapest in the 

 end, to undertake this course with all the sheets of the new series 

 as they come out, irrespective of the demand upon each par- 

 ticular one. If these could be produced at the price of \s., 

 instead of 4^. , a brisk demand would be at once created. The 

 public must, however, do its share. We have already stated 

 that the index-sheet is issued as an experiment. If the issue is 

 well supported, this will furnish a great encouragement to the 

 Stationery Office to follow the good course it has begun. 



A GRAPHIC method ot determining the focal lengths of lenses 

 and mirrors is described by Dr. E. H. Barton in the Philo- 

 sophical Magazine. For a concave mirror, cut off the distances 

 of the object and the image respectively on the two axes of 

 Cartesian coordinates, and join the two points. Two separate 

 observations will give two lines intersecting in a point equi- 

 distant from the two axes. The coordinates of this point are 

 both equal to the focal length required. The measurements 

 may be controlled by another observation, and the line now 

 obtained should intersect the other two in the same point. A 

 line passing through this point and rotating about it will cut oft 

 in succession all the possible values of the conjugate focal 

 distances. For a convex lens, the point is situated in the right 

 hand lower quarter, for a convex mirror in the left-hand lower, 

 and for a concave lens in the left-hand upper quarter of the 

 plane of coordinates. 



On no other frequented trade route are vessels so liable to be 

 obstructed by drift ice as in that portion of the South Atlantic 

 lying to the east of Cape Horn and the Falkland Islands. A 

 chart just issued by the U.S. Hydrographic Office, to show the 

 limits of the enormous ice fields encountered by mariners in 

 those waters, will therefore be of great service. The chart also 

 gives for the months of March, April, and May the isotherms or 

 lines of equal temperature of the surface water. It is stated, 

 however, that these lines are of doubtful value to the navigator 

 in announcing the proximity of ice, as practical experience 

 has shown that the temperature of the surface is little affected 

 thereby. The report of Captain Macmillan, of the ship Dud- 

 hope, is especially interesting in this connection : ' ' Careful ther- 

 mometric observations of air and water were regularly taken, 

 but our approach to ice, always from windward, was not once 

 indicated by any appreciable change of temperature, in either 

 air or water. On passing to leeward of the bergs, a fall of a few 

 degrees was generally observed in the air. On one occasion we 

 passed within a cable's length of a berg, and found the tempera- 

 ture to be the same there as at several miles' distance. This 

 would go to show that in thick weather — or in any other — even 

 temperature and thermometer at normal height should not be 

 accepted as a trustworthy guarantee of immunity from ice. Care 

 and a most vigilant look-out are the only trustworthy safeguards. 

 To depend on the thermometer would mean disaster, as I am 

 convinced that a ship would be too close to the ice to extricate 

 herself by the time the thermometer would indicate its pre- 

 sence." 



Recent progress in the chlorination process for the extraction 

 of gold from its ores lies mainly in the direction of improve- 

 ments in the mechanical appliances which are used. The re. 

 peated efforts to prevent the oxidising action exercised by chlorine 

 on the unroasted sulphides in ores, by adding salts such as nitre 

 to the mixture, have now apparently been abandoned. Most 

 chemists have always regarded the hopes of the experimenters 

 in this direction as chimerical. Among the mechanical improve- 

 ments of the last year have been the enlarging of the lead-lined 

 steel barrels used in Western America. As now made, these 



