190 



SCIENCE. 



[Vol. V., No. 109. 



place your elbows at a convenient distance apart. 

 This position evidently leaves a space between your 

 chest and the back of the chair equal in length to 

 your fore-arms, which are extended horizontally. 

 Miss Lulu now takes a position beside you, and, hold- 

 ing her body back, simply places the palmar surface 

 of her hand on the back of the chair on the side to- 

 wards your body. After a few moments she seems 

 to make the effort to detach her hand from the chair, 

 which latter you are privileged to push forwards. 

 The force at work, however, is too strong for you, 

 and both yourself and the chair are carried back- 

 wards, without her hand having changed its position. 

 The chair being a cane-backed one, it is evident 

 that she could in no way gain a hold upon it, and the 

 back of her hand never could come in contact with 

 your chest, as the spanning of such a distance would 

 at once be detected. 



Professor Newcomb's conclusions, after having 

 witnessed the test of lifting a chair with some one 

 sitting in it, are to me far from satisfactory. I 

 saw the girl lean over an ordinary chair, with a man 

 weighing over two hundred pounds sitting in it, 

 and placing the palmar surfaces of her hands on 

 the outer sides of the rear uprights near their 

 middles, and without any contraction of the muscles 

 of the arm or fore-arm, or increase of pulse 

 (remained at 80) or respiratory effort, or change 

 of countenance due to exertion, so far lift that chair 

 and its heavy contents from the floor as to compel 

 the latter to get out of it; and this without fractur- 

 ing any of the bones of her upper extremities, or the 

 sides of the chair. The simplest computation will 

 prove that the lateral pressure required must be enor- 

 mous in order to get a hold, and prevent such a weight 

 absolutely slipping between her hands when the up- 

 ward force comes to be exerted. 



R. W. Shufeldt, U.S.A. 



Fort Wingate, New Mexico, Feb. 19. 



THE MICROSCOPE IN GEOLOGY. 



Many persons have heard that the micro- 

 scope, everywhere recognized as indispensable 

 in the investigation of organic nature, has also 

 recently been made use of in geology ; but 

 very few have any distinct notion of the sort 

 of problems to which it can there be applied, 

 or of the way in which it can contribute toward 

 their solution. The determination of the dif- 

 ferent minerals which compose very fine grained 

 rocks may doubtless appear, even to many 

 geologists who have been accustomed to deal 

 with only great areas and mountain masses, a 

 matter of small importance ; and they often 

 fail to see that the methods which render such 

 a determination possible, are capable, if prop- 

 erly employed, of throwing much light on some 

 of the most difficult questions with which they 

 have to deal. 



The microscopic study of rock-sections is 

 one of difficulty, and indeed quite discour- 

 aging to a beginner who attempts it without 

 proper guidance, no matter how familiar he 

 may be with mineralogy, or with the use of the 



microscope in other fields of research. This 

 fact, coupled with the newness of the branch, 

 sufficientl} T accounts for the number of workers 

 in it still being so small in this country, which 

 presents unrivalled opportunities for its cul- 

 tivation. 



Although the idea of preparing rocks in 

 transparent sections for the microscope ori- 

 ginated with an Englishman, the fruitful line 

 of research to which it gave rise has since 

 been almost exclusively cultivated in German}'. 

 Here the seed fell into soil made already fer- 

 tile by the labors of older geologists, and 

 sprang at once into a strong and rapid growth. 

 The keen perception and great energy of Zirkel 

 first made known the microscopic appearance of 

 the common rock-forming minerals, as well as 

 discovered the wide distribution of others before 

 considered rarities. Vogelsang, not contented 

 merely to observe, was able to draw from his 

 studies the most suggestive conclusions, which 

 he substantiated by ingenious and delicate 

 experiments. It is, however, to Rosenbusch 

 that the development of petrography as a sci- 

 ence is most largely due. In his work, pub- 

 lished in 1873, he showed in a masterly manner 

 how what had been learned of the optical prop- 

 erties of different crystals, especially their 

 action on polarized light, could be applied to 

 their identification in thin sections, thus ren- 

 dering a rigid microscopic diagnosis for the first 

 time possible. From this time on, the inter- 

 est in this branch of investigation became in 

 Germany very general, and its growth propor- 

 tionately rapid. The attainment of the long- 

 desired separation of rock constituents, even 

 when of the smallest size, by means of solu- 

 tions of high specific gravity, and the perfec- 

 tion of many micro-chemical reactions of great 

 precision, followed each other in quick succes- 

 sion, until to-day the accuracy and beauty of 

 petrographical methods are hardly second to 

 those found in any other branch of natural 

 science. 



The geologists of other countries on the con- 

 tinent, especially in France and Scandinavia, 

 soon perceived the value of the German work, 

 and early availed themselves of its results to 

 start similar investigations in their own coun- 

 tries. It is a surprising fact that the apprecia- 

 tion of it among English-speaking people has 

 been so slow, that not one reliable text-book 

 on the subject of petrography exists in the 

 language of the man who gave the first im- 

 pulse to its modern development. Any knowl- 

 edge of the subject in America is recent, dating 

 from the publication of Zirkel' s ' Microscopical 

 petrography' in 1876. How steadily the inter- 



