42 



THE AMERICAK MONTHLY 



[March, 



The first important petrographic 

 work done in this country was by 

 Zirkel for the Fortieth Parallel Sur- 

 vey, but at the pesent time there are 

 nearly a score of investigators, most of 

 whom were students of either Zirkel 

 or Rosenbusch, actively engaged in 

 research . 



Although it will hardly be admitted 

 by microscopists generally, neverthe- 

 less the microscope appears to be a 

 much more important instrument in 

 the study of inorganic than organic 

 nature. The molecular structure 

 brought about by vital force is not 

 of such a sort as to impress its char- 

 acter upon transmitted light. On 

 the other hand the inorganic bodies 

 which occur in nature as minerals 

 have each its peculiar form of crystal, 

 and each so modifies the light trans- 

 mitted through it as to indicate the 

 system of its crystallization. The 

 great value of the microscope in the 

 study of petrography arises largely 

 from the facility it afibrds for observ- 

 ing minerals in transmitted light. 



The speaker remarked that it would 

 not be appropriate at this time to en- 

 ter into a discussion of the great geo- 

 logic problems, the solution of which 

 depends so largely upon the revela- 

 tions of the microscope. He had. 

 however, brought with him several 

 microscopes of French and German 

 patterns showing the modifications 

 which especially adapt them to petro- 

 graphic work. Besides the ordinary 

 parts of a compound microscope, the 

 i nstrumen ts exhibited had each a 

 polarizing apparatus, a rotating stage, 

 and a l&ns beneath the stage, so that 

 when desirable investigations mav be 

 made in converging light. It is very 

 important to ha^■e the objective so ad- 

 justed to the axis of the stage that 

 v^^hen the latter is rotated the object 

 under examination may not be turned 

 out of the field of vision. In the 

 German instruments made by Voigt 

 and Hochgesang in Gottingen, and 

 Fuess in Berlin, either the objective 

 or the stage is adjustable to the other, 

 but in the Nachet microscope a much 



better plan has been adopted of sep- 

 arating the tube into two parts, and 

 supporting the objective from the ro- 

 tating stage in such a way that both 

 rotate together about the same axis. 

 The French instrument has a great 

 advantage over all others in the use 

 of a spring clamp for fastening the 

 objective to the tube, and an arrange- 

 ment for swinging the analyzer in and 

 out of the tube, which greatly ft^cili- 

 tate rapid examinations. 



The speaker then exhibited some 

 small specimens illusti'ating the way 

 in which thin sections of rocks are 

 prepared for microscopical investiga- 

 tion. The rock chips are first ground 

 perfectlv flat upon one side and then 

 cemented bv Canada balsam upon a 

 thick slip of glass. The other side is 

 then ground down, the preparator 

 holding the slide by the slip of glass 

 to which it is cemented. The coarse 

 grinding is done upon a cast-iron plate 

 b^' hand or upon rotating disks of the 

 same metal, using in both cases No. 

 60 emery. The section is finished 

 upon a smooth glass plate with FFFF 

 best English washed emery. It is 

 then detached by heat from the thick 

 slip, transferred to a thin transparent 

 one and permanently mounted in Can- 

 ada balsam. 



Occurrence of Red Snow. 



I was interested in the paper 

 recently read before the Biological 

 Society on the ' Chlainydococcus 

 nivai/'s,'' which tinges the Arctic 

 snows red. 



All the observations of naturalists 

 point directly to the higher mountain 

 slopes as the birthplace of the plant 

 from which the red deposits origi- 

 nate, although they are mentioned by 

 many of the relators as being found 

 near glacier clifts, as at Beverly and 

 Baffin's Bay, in the Arctic seas, by 

 Kane and Ross. In all countries 

 where glaciers exist, an exuberant 

 o-rowth of lichens immediately fol- 

 lows the denudation of the rocks from 

 which a high midsummer tempera- 



