5S2 



NATURE 



[April 21, 1898 



and crystallographic relationships, in place of the notes 

 appended to each group in the earlier editions ; and the 

 crystal class to which each mineral belongs is defined. 

 Thus the book, while preserving the form which is now 

 so familiar to every student of mineralogy, is in reality 

 much more of a treatise on mineral chemistry than it was 

 before. In fact, it may be regarded as the most useful 

 text-book on this subject available for a student. The 

 researches of Penfield and of Clarke have thrown a 

 flood of new light upon the chemistry of minerals, and 

 Prof. Groth is, as is well known, never slow to in- 

 corporate the latest results of science in his books. 



These tables are indispensable to the student, and of 

 immense use to the systematic mineralogist : to the 

 latter especially in those passages which express the 

 critical views of an author of unrivalled experience and 

 judgment upon minerals of obscure composition. 



The following examples, chosen almost at random, will 

 suffice to illustrate the modifications which have been 

 introduced into the old classification. Pyrrhotite is now 

 FeS, and is placed in the Wurtzite group ; Lorandite, 

 the new sulpharsenite of thallium, is placed with Mi- 

 argyrite ; Ilmenite has been removed from the Hfematite 

 group of oxides, and is classed with Pyrophanite as a 

 titanate of iron, in consequence of recent observations 

 by Penfield. 



But it is needless to multiply examples. The book 

 should be the handy companion of every mineralogist. 

 It is only necessary that in this, as in former editions, 

 the reader should bear in mind that rational formulae in 

 mineralogy are never free from the taint of speculation. 



LETTERS _ TO THE EDITOR 



[The Editor does not hold himself responsible for opinions eX' 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 vanuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.'\ 



Sub-Oceanic Terraces and River Valleys of the Bay 

 of Biscay. 



Will you allow me to add to my communication, which 

 appeared in Nature of March 24, regarding the results of 

 investigations carried out by means of the Admiralty charts 

 beyond the coast of the British Isles, a still further statement 

 regarding the Bay of Biscay ? The results are not less remark- 

 able and suggestive of great changes of level than are those in- 

 dicated by the soundings to the west of the British Isles, which 

 I hope to describe in full at the meeting of the Victoria In- 

 stitute on May 2. We have the same general physical con- 

 tours : first, the Continental shelf, or platform, stretching out 

 for a distance of over 100 miles along the northern edge of the 

 bay, but narrowing southwards till along the coast of Spain it 

 contracts to an average breadth of twenty to thirty miles. At 

 its outer margin the shelf breaks off at the 200-falhom line in 

 a grand escarpment, continuous with that of the British plat- 

 form, and descending with a more or less steep descent to 

 somewhat over the 1200-fathom line. Off Cape Ortegal the 

 escarpment is almost precipitous, for the total descent of 

 7000-8000 feet is effected within a horizontal space of about 

 four miles ; from the base of the escarpment the'; ocean-floor, 

 formed of calcareous ooze, stretches gradually away to depths 

 of 2600 fathoms and upwards. The escarpment forms the 

 natural boundary between the region of Globiiferina ooze and 

 the Continental platform of sand, mud and gravel with shells. 



Lastly, the old channels of several of the river valleys can be 

 distinctly traced when they approach and intersect the grand 

 escarpment. This is remarkably so in the case of the Loire, 

 the' Adour, the Las Cubas and Caneiro rivers. The Adour 

 passes out to the deep ocean through a continuous deep canon 

 or gorge of 100 miles in length — marked on the Admiralty 

 chart at its upper end as '* Fosse de Cap Breton." At a dis- 

 tance of thirty miles from the coast the bed of the canon is 

 about 3174 feet below the general level of the Continental shelf, 

 and at a distance of sixty-two miles it descends to a depth of about 

 5442 feet below the same level. The caiion itself can ba distinctly 



NO. i486, VOL. 57] 



traced to the depth of 9000 feet (1500 fathoms), where it opens 

 out on the oceanic floor, representing the feature known by 

 American geologists as "the base level of erosion" — in other 

 words the limiting depth of river-erosion at the time when the 

 land was elevated, and indicating the extent of the elevation as 

 compared with the ocean level of the present day. The " em- 

 bayments" of the other rivers mentioned can be traced to about 

 the 1200-fathom contour. Such concurrence of evidence as 

 regards the greatfuplift, presumably reaching its maximum at 

 the commencement of the Glacial Epoch, is conclusive in its 

 testimony. The British and Continental platform was probably 

 referable to the Mi o-pliocene period. Edward Hull. 



April 19. 



Dust Fog in the Canaries. 



In amplification of the notice published in Nature con- 

 cerning the dust shower encountered by the Roslyn Castle off 

 the west coast of Africa, I have the honour of sending you the 

 following description of the phenomenon as experienced in the 

 city of Laguna (Teneriffe), that I have received from Prof. 

 Calvo, keeper of the Meteorological Station. I send also a 

 summary of the meteorological register. 



AuGUSTO Arcimis. 



Inst'tuto Central Meteorologico, Madrid, April 14. 



From the first hours of the evening of February 15, there was- 

 observed a sensible but light fog ; neither the strength of the 

 wind (E. gentle breeze) nor any other phenomenon indicated 

 that the supposed condensed vapours could be African dust 

 transported by the air. As the night advanced, the force of the 

 wind increased, until it reached the value of a moderate gale. At 

 about 5 a.m. on the i6th, some large drops of rain fell, but were- 

 inappreciable in the rain gauge. For a very short time the wind 

 subsided, by and by becoming again a gentle breeze during the 

 day and blowing due E. The fog became more dense, causing 

 depression and a disagreeable feeling produced by its dryness. 

 The sun, on account of its light being pale and feeble without 

 the usual rays, was confounded with the moon ; and it 

 reminded one of the light of a voltaic arc seen through a frosted, 

 glass. The flame of a match appeared, with a very markedi 

 violet hue. The drinking waters became salty and coloured as 

 by oxide of iron. The dust was grey and extremely fine, and 

 deposited itself on every object. 



On February 19, from the early morning the sky was again, 

 clear and transparent, and the wind was blowing from the N. 

 and N.W. 



Meteorological Observations taken at Laguna (Teneriffe), during' 

 the Dust Fog, in February 1898. 



