796 



[N. S. Vol. IV. No. 100. 



two series, carefully distinguished, witli the 

 same person by name or initials. 



When using one figure, the other should be 

 covered. 



The results, whether from one person or 

 from many, may be sent to the undersigned, 

 who will receive them with thanks. Results 

 from those who know what the illusion is and 

 what to expect need not be sent, except in cases 

 of persons who do not get the illusion at all, or who 

 only get it for one of the figures. 



Any known defects of eye-sight should be 

 reported; also indications of tastes or pursuits, 

 as of architects, artists, etc., likely to modify 

 the results. 



I should also be glad to be referred to any 

 literature which seems to touch upon this illu- 

 sion. J. Mark Baldwin. 



Peinceton, N. J., October 7, 1896. 



LE CONTE'S elements OF GEOLOGY. 



To THE Editor of Science : I read with great 

 interest Mr. Gilbert's review of Le Conte's 

 Elements of Geology, in Science, No. 95. 

 Having used the book in the classroom I can 

 heartily approve every word of commendation 

 in that review. It is therefore not in the spirit 

 of captious criticism that I venture to point out 

 two or three weaknesses in the book which ex- 

 perience brought to my attention. I do this 

 because Le Conte's Elements is *a Textbook,' 

 and, if Prof. Simonds' calculations* be correct, 

 only a portion of the teachers of geology are 

 investigators who would detect from their 

 own researches either the strong points or the 

 weak ones of the book. It may be urged that 

 geographical considerations have much weight 

 in my case ; yet no more weight with me than 

 would naturally be felt with any other teacher 

 of geology between Lake Huron an^ the Rocky 

 Mountains north of the 40th parallel. 



I desire to call attention only to the follow- 

 ing points : 



1. Artesian wells are discussed within the 

 limits of a single page (p. 76). Several wells 

 are named, bored in widely separated localities, 

 and the depth reached in each is stated. As 

 the examples given are among the deepest bor- 



* Geology in the Colleges and Universities of the 

 United States. Frederic W. Simonds, Science, 

 Oct. 2, 1896, p. 497. 



ings in the world the natural inference is that 

 an abundant flow can be found if the contractor 

 only bore deep enough. Such an inferencej is 

 not only wholly wrong in its practical aspects, 

 but it is not the result of good geological 

 reasoning. 



In many portions of the country, notably in 

 the northern belt of states from Ohio to the 

 Rocky Mountains, much attention is given to 

 the subject of Artesian water supply for do- 

 mestic uses, power and irrigation. Its geology 

 is important. Qualifying conditions must al- 

 ways be weighed and understood. The char- 

 acter of the water is an important factor, since 

 within certain limits its chemical composition 

 determines its usefulness. Within this area the 

 geological character of the formations pene- 

 trated have become pretty well known to the 

 depth of half a mile. Yet with all this scien- 

 tific and economic interest within so large a 

 portion of the United States, there is no aid in 

 Le Conte's Elements for the teacher in the 

 presence of a class anxious to take away some- 

 thing practical, or for the general reader seek- 

 ing information as to where and how he shall 

 proceed to obtain artesian water, although he is 

 told with much detail how to find the epicen- 

 trum and focus of an earthquake. 



2. In the subject of historical geology two or 

 three points command attention. The Archean 

 era and system are first to be noted. As the 

 classification on page 295 is compared with 

 that of working geologists, the labors of the 

 last 22 years .within the field of Pre-Cambrian 

 geology receive but little recognition. In 1874, 

 in his 2d edition of the Manual, Dana recog- 

 nizes the ' Primordial or Cambrian, ' and places 

 beneath it the Archean with its sub-divisions, 

 Laurentian and Huronian, In 1896 LeConte 

 does the same. Teachers and general readers 

 in geology wpuld receive much more help from 

 the conclusions of the Geological Conference in 

 Washington, , January, 1889, in which confer- 

 ence Mr. Gilbert was himself a leading figure, 

 had its results touching these basal formations 

 been used by Professor LeConte. 



The Lake Superior basin, with its southerly 

 borders, has been for years the center of interest 

 to students ©f petrographic and historical ge- 

 ology. The work of Irving, Van Hise and 



