Oct. 2, 1879] 



NATURE 



527 



estimate is given at six times the amount of the soluble 

 matters. This produces over the whole globe an amount 

 of denudable matter equal to 600 tons a square mile per 

 year. Going back in all time at this rate, and allowing 

 for coast erosion, glaciers, &c., the ten miles of sedi- 

 mentary strata must have occupied 526 millions of years 

 in accumulation. The author readily disposes of Sir 

 William Thomson and tidal retardation, and his limits of 

 time. His calculations are "fallacious through leaving 

 out agencies that we know are at work, and which the 

 calculations I have submitted bring out in greater force." 

 He admits that he does not know how deep the sedi- 

 mentary strata are, and it does not appear to have entered 

 into his calculation that there was from the beginning 

 carbonate of lime and sulphate of lime in sea-water, that 

 the rainfall must have varied during geological time, and 

 that the denudation of the surface has brought rocks of 

 different solubilities within the reach of rain. Again, his 

 calculations are vitiated by the fact that a vast amount of 

 water percolates through many kinds of strata and does 

 not come into the neighbouring river valleys, and that 

 there are great artesian collections not in communication 

 ■with the sea. In fact, one of the great problems of the 

 day is to explain what becomes of vast quantities of such 

 water. We must leave the author to settle with Prof. 

 Geikie all those interesting calculations which are founded 

 upon the hard and fast lines of uniformity. 



There are some remarkable statements in this essay. 

 Thus Hutton "laid the foundation of our present know- 

 ledge of physical geology." He gave us the grand method 

 of geological study, but certainly many of the facts were 

 well known before his time, and others have had no rela- 

 tion to his researches or method. Then there is the 

 curious notion resuscitated as " a result of the Challenger 

 expedition," that the calcareous portions of the dead 

 foraminifera are dissolved by the carbonic acid in the 

 sea before they reach the bottom. There are calcareous 

 organisms in the red clay nevertheless, but perhaps they 

 liave escaped. It was a pretty idea that Thomsonian 

 myth of the nymph Globigerina sinking into the arms of 

 Neptune, and falling blushing and nude on to the abyssal 

 Ajor, turned, like Adam, into red clay. But myths are not 

 ience, though there is a science of myths. In his paper 

 n the Challenger discoveries, the red clay is a terrible 

 .cubus; and the following quotation will show that the 

 author's physics are sometimes confused : — 



"Now, this lowering of the bottom temperature over 

 such immense areas is certainly a remarkable and unex- 

 pected fact, and shows that the secular cooling of the 

 earth must be extremely slow, as to all appearances 

 contact with the bottom does not in any case appreciably 

 influence the temperature of the bottom water." 



As a matter of fact the author is wrong in giving the 



Porcupine credit for discovering the great extension of 



the globigerina ooze, and he is not justified in calling the 



odules of the red clay peroxide of manganese, for there 



, much iron in them, and he has been misled as to their 



ast quantity. 



One of the funny notions of some scientific thinkers meets 

 with no favour from Mr. Readc, whose geological know- 

 edge is practical as well as theoretical. They consider 

 hat because the older rocks contain nothing like the 

 resent red clays, &c., of the ocean floor, that the oceans 

 avc always been in their present positions. Mr. Reade 



points out that the first proposition is not that proven, and 

 the distribution of animals and plants and the fact that 

 the bulk of the strata on land are of marine origin are 

 opposed to the hypothesis. He very properly waits with 

 the rest of the world for the final publication of the 

 Challenger facts, and in the meantime enjoys the theories. 

 In the last of his papers the author begins by asserting 

 that "The geological history of the globe is written only 

 in its sedimentary strata." He proceeds to state that 

 limestone rocks have been in process of formation from 

 the earliest known geological period, and measures the 

 absolute quantity of carbonate of lime in the sedimentary 

 rocks. The depth of these he tries to estimate by 

 borings ! by the caiions of Colorado ! ! by the denudation 

 of Suliven ! ! ! by the amount of downthrow of faults and 

 the thickness of the strata of mountain masses. In fact 

 by every way except the right one, by the process of the 

 gathering section. He considers that one mile is about 

 the average thickness of the sedimentary crust, and that 

 one-tenth of it is limestone, or 528 feet enveloping the 

 globe. On the strength of former calculations, he believes 

 that one foot of this universal stone would take 1,139,032 

 years to accumulate, or that in round numbers — a million 

 or two are nothing in the way of argument — the whole 

 occupied 600 millions of years, and this is the minimum 

 age of the earth since the first Laurentian sediments. 

 Mr. Reade has given a number of most valuable facts in 

 his book relating to water flow and soluble matters, and 

 his readers will pardon him for running a little riot in his 

 theories. 



LETTERS TO THE EDITOR 



[The Editor does not hold himself responsible for opinions expressed 

 by his correspondents. Neither can he undertake to return, or 

 to correspond with the writers of, rejected manuscripts. No 

 notice is taken of anonymous communications. 



[The Editor urgently requests correspondents to keep their letters as 

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 is impossible otherwise to ensure the appearance even of com- 

 munications containing interesting and novel facts.\ 



Colour-Blindness . 



The normal perception of colour, according to the Young- 

 Helmholtz theory, depends on the coexistence in the retina of 

 three sets of nerves capable of conveying three distinct sensa- 

 tions, and excited most strongly by ether-vibrations of three dis- 

 tinct rates. The simple sensations conveyed to the sensorium 

 by these nerves are known as primary colours (red, green, violet), 

 and all other colour-sensations are produced by combined action 

 of at least two out of the three in various proportions. Dichromic 

 vision is explained by the absence of one of the three primary 

 sensations, in most cases that which corresponds to the long-wave 

 sensation, red. 



Dr. Pole points out, in his article (Nature, vol. xx. p. 477) 

 that this hypothesis does not account for the most typical condi- 

 tion of colour-blindness, such as he describes in his own case. 

 It seems to me, however, that a slight modification of the theory 

 is all that is necessary to obviate this objection. 



I would suggest that the nerve-fibres are not always most 

 strongly excited by exactly the same wave-length, or, in other 

 words, that the primary colours are not precisely identical in all 

 normal eyes. There may well be small differences, and it is 

 only when the divergence is creat that the differences are appreci- 

 able and "colour-blindness is noticeable. 



Such a purely personal and subjective question as whether all 

 normal eyes are similarly affected by similar light-waves is, of 

 course, difficult to determine ; but at least it is possible that a 

 very slight structural difference in the nerve-ends of the retina 

 may occur, sufficient to render them excitable chiefly, let us say, 

 by the more rapid waves at the sodium line D, instead of the 

 slower waves of line I!. 



