l>36 THE AMERICAN MONTHLY [July 



more or less opal or uncrystallized silica which contains 

 water, so that analysis shows that chalcedony contains 

 from 0.3 to 2.5 per cent or more of water. 



In this section, when examined in polarized lijfht, the 

 needle-like radiating- crystals of the quartz are seen to form 

 circular areas, in which the successive layers of g-rowth 

 can be well seen. These areas represent in section the 

 rounded elevations of the free surface. Each circular area 

 is composed lof a very g-reat number of crystals, which 

 g-ive it the radial character. These areas interfere with 

 each other more or less, but an examination of their 

 ccMicentric banding- as seen with the polariscope ena- 

 bles one to trace out the different stag-es of the forma- 

 tion of the mineral. At the edg-es representing- orig-inal 

 free surfaces some well-formed crystals of quartz have 

 developed and encrust the edg-e of the section. The same 

 reg-ion shows the enclosure of foreig-n matter, g-iving- a 

 banded appearance in ordinary lig-lit. This is essentially 

 the structure of ag-ate, which is simply banded (and g-cner- 

 ally colored) chalcedony. Carnelian or sard is likewise a 

 varietv of chalcedony of a red or yellow color (not banded). 



Amos P. Bkown. 



NEW PUBLICATIONS. 



Biolog;ical lectures delivered at the Marine Laboratory 

 at Wood's Holl. (iinn & Co., ]>}). ISS; con>liUiU's a very 

 interesting- volume. 



Microscopic Internal Flaws iutlucing^ Fracture in Steel. 

 \ly Thomas Andrews, F. R. S., F. C. S., M. Inst. C. E., 

 etc. Svo, pp. 52. ' London : 10. and I-^. N. Spon. 189().) 



xV paper of considerable import. mce to Civil Eng-ineers, 

 rejn-inted from Fng-ineering-, on Microscopic Internal 

 Flaws in Steel, Railway Locomotive and Straig-ht Axles, 

 Tyres, Rails, Steamship Pr()j)eller Shafts, and Propeller 

 Crane Shafts, and other Shafts. Bridgfe (iirder Plates, 

 Ship Plates, and other Eng-ineering- Constructions of Steel. 

 There are 30 micro, lig-ures showing- internal defects. 



