February 9, : 



SCIENCE. 



11 



Assuming the width of the seas and continents to 

 be 3,900 miles, the rigidity of the earth to be 3 X 10**, 

 as above, and the range of the tides to be 80 centi- 

 metres, Mr. Darwin computes and gives tables of the 

 slopes, real and apparent, of the land at various dis- 

 tances from the coast. Such deflections, he thinks, 

 might actually be observed at points near the coast, 

 and the measurements thus obtained might possibly 

 serve as a basis for computing a more trustworthy 

 value of the earth's rigidity than we now possess. 



Under the conditions above assumed, the amplitude 

 of vertical displacement between high and low tide 

 is 11.37 centimetres on the land at the coast. 



" As long as h I" — i.e., the semi-range of the tide 

 multiplied by the width of a sea or continent — " re- 

 mains constant, this vertical displacement remains tlie 

 same; hence the high tides of ten or fifteen feet which 

 are actually observed on the coasts of narrow seas 

 must probably produce vertical oscillations of quite 

 the same order as that computed." E. H. Hall. 



LETTERS TO THE EDITOR. 



>iis are requested to be as brief as possible. The 

 writer's name is in all cases required as proof of good faitli.^ 



Age of the rocks on the northern shore of Lake 

 Superior. 



Peemit me, through the medium of your journal, 

 to correct a mistake which Prof. N. H. Winchell 

 has made (Tenth ann. rep. surv. Minn., p. 125) in 

 stating that I regard the trap and sandstone of Lake 

 Superior as Huronian. 



Up to the present time I was not in a position, 

 never having examined them, to express any opinion 

 about the Lake Superior formations referred to. 



During the past summer I have somewhat closely 

 examined these around the whole of the Canadian 

 shores, from Prince Arthur's Landing to Sault St. 

 Mary, including the shores of Thunder Bay, Black 

 Bay, and Nipigon Bay and Straits. I spent two 

 months in this examination, travelling from point to 

 point in a small boat. 



My opinion now, respecting the character and age, 

 — within certain limits — of these rocks is very de- 

 cided, and is as follows: — ■ 



They occupy the geological interval elsewhere filled 

 by those divisions of the great lower paleozoic system 

 which underlie the Trenton group. Various consid- 

 erations point to the Potsdam and Primordial Silu- 

 rian (Lower Cambrian) as their nearest equivalents. 

 They are entirely unconformable to, and physically 

 distinct from, the Huronian. They are divisible on 

 the Canadian shores into two, perhaps three, groups, 

 between which there may be slight unconformities. 

 These, however, are quite likely only such as might 

 result from the intermingling of ordinary sediment- 

 ary strata with irregular layers of erupted volcanic 

 material, molten, muddy, and fragmentary ; the whole 

 being subsequently, and even during their accumula- 

 tion, further disturbed by faulting, and the irruption 

 of igneous dykes and masses. 



To my mind, there can be no doubt as to the nature 

 of the causes which have built up the vast masses of 

 strata, which now, together with ordinary sediment- 

 ary layers, form the so-called upper copper-bearing 

 rocks of Lake Superior. They are essentially vol- 

 canic, subaerial, and subaqueous formations, and in 

 every sense analogous to the wide-spread tertiary 

 volcanic rocks of Australia and other regions. The 

 only differences are their greater antiquity, and the 

 consequent greater changes and modifications they 

 have undergone through the operation of long-con- 



tinued metamorphic agencies, disturbance, and de- 

 nudation; though these changes are far less than 

 those which the rocks of the same age, and to some 

 extent similar origin, have undergone in eastern 

 America and in Britain; and in this they correspond 

 with the higher fossiliferous groups in the respective 

 regions. 



The groups in ascending order are, — 



1. Black shales, flinty and argillaceous, banded 

 chert, with black dolomites and beds of fine-grained 

 dark-gray sandstone with mica in the bedding planes; 

 the whole interbedded with massive diabase or 

 dolerite, often columnar, the columns vertical. — Pie 

 Island, McKay's Mountain, Thunder Cape, etc. 



2. Red conglomerates, red and white and green 

 mottled shales, red and white sandstones and dolo- 

 mites ; no gray or black beds. At perhaps a hundred 

 and fifty or two hundred feet from the base, these 

 become interstratified with massive beds of volcanic 

 material, amygdaloids, melaphyres, tuffs, etc., mak- 

 ing many thousand feet of strata. — East shores 

 of Black Bay, Nipigon Strait, St. Ignace and other 

 islands, Miohipicoton Island, Gargantua, Mamainse, 

 etc. 



3. The Sault St. Mary sandstones. These may be 

 only the upper part of 2, without any intermingling 

 of volcanic material. The exposures on the Cana- 

 dian side are too fragmentary and isolated to decide 

 this. In any case the St. Mary sandstones are not 

 younger than Chazy (Cambro Silurian), but in the 

 absence of fossils it is impossible to correlate the 

 Lake Superior groups exactly with any one of the sub- 

 divisions of the New York or the Atlantic coast series. 

 This, however, is no sufficient reason for inventing 

 and adopting new and unknown names for them; 

 and I prefer to call them all Lower Cambrian, which 

 includes Potsdam and Primordial Silurian. There 

 is, at present, no evidence whatever of their holding 

 any other i^lace in the geological series. Through 

 overlapping and faulting, all three divisions are 

 found locally in contact, both with Huronian and 

 with Laurentian rocks. The dips are generally south- 

 eastward, but vary greatly in amount, those of divis- 

 ion 2 being often locally much higher than any ob- 

 served either in divisions 1 or 3. A. K. C. Selwtn. 

 G-eol. and nat.-hist. survey of Canada. 



Ottawa, December, 1882. 



Movement of the arms in 'w^alking. 



Every man has observed that the tendency to 

 swing the arms while walking is a most natural one. 

 The action is rhythmical, the anterior and posterior 

 extremities of opposite sides of the body moving in 

 unison. It is also involuntary, being performed most 

 readily when thought is not bestowed upon it. When 

 voluntarily suspended, as in the American army, it 

 gives an air of 'stiffness.' 



In view of these facts, does it not seem that the 

 statement of Prof. J. D. Dana (Cephalization; Amer. 

 joiirn. sc, xli. 1866, p. 167), sanctioned by Dr. T. 

 Gill (Classif. families of mammals, 1872, p. 50), — 

 namely, that "Man stands alone among mammals 

 in having the fore-limbs not only prehensile, but 

 out of the inferior series, the posterior pair being 

 the sole locomotive organs," — must be somewhat 

 modified? Have we not at least a ghost of a pre- 

 existing function? Does man walk by means of his 

 feet and legs alone ? Fkedekick W. Teue. 



Cleaning birds. 



Wlien obliged to wash bird's, collectors will find it 

 an advantage to use salt and water instead of plain 



