922 SUMMARY OP CURRENT RESEARCHES RELATING TO 



avail themselves of such an important aid. The use of the Microscope 

 alone will in future produce but little that is new ; but its possibilities 

 in geology, when intelligently employed in connection with the most 

 detailed and careful field-work — the necessity of which has been 

 increased, not diminished, by its introduction — cannot be easily over- 

 rated. 



What pal8Bontology has done for the fossiliferous deposits, this, 

 and even more, the Microscope must do for the crystalline rocks. 

 The less altered forms of igneous masses have thus far been almost 

 exclusively studied ; and, although they still have much to teach us, 

 it is not by their investigation that the Microscope is destined to 

 yield its greatest assistance to geology. The changes, structural and 

 chemical, which go on in rocks after they are first formed, leave 

 behind them more or less distinct traces which it is the special pro- 

 vince of the Microscope to follow out and interpret. ... It is by 

 dealing with such problems as Lessen, Renard, and Lehmann in 

 Europe, and Wadsworth in this country [U.S.A.] have especially 

 pointed out that the Microscope in geology can in future render its 

 best service. The manner in which this can be accomplished is by 

 the patient following, step by step, of unchanged rocks into their most 

 completely altered equivalents, and carefully comparing the condition 

 of each constituent at every point. In this manner the succession of 

 changes which they undergo may be as completely worked out as 

 though we could see the process actually going on before our eyes. 

 . . . What is especially to be desired are detailed studies of many 

 small areas where the same rock, whether eruptive or sedimentary, 

 can be traced from its original form to its more altered state and a 

 comparison of the results obtained in each. This Lessen has recently 

 attempted for the southern Hartz, and has thereby indicated what is 

 perhaps the most promising field for microscopic work in geology." 



Application of the Microscope to Practical Mineralogical 



Questions.* — In examining an argentiferous mineral which was found 

 in Wales, and known there as " blue stone," it became desirable to 

 determine whether the mineral was a definite double sulphide of lead 

 and zinc, or whether it was a fine mechanical mixture of the two well- 

 known minerals galena and blende. Prof. Tichborne found that on 

 gradually powdering the mineral and examining it from time to time 

 under the Microscope, a point was at length reached when half the 

 particles became transparent and transmitted light, whilst no amount 

 of powdering would render the other particles transparent. To try 

 such an experiment it was necessary to view with very strong trans- 

 mitted light (a 1/2 in. object-glass) and to cut off all reflected light. 

 From this experiment he came to the conclusion that the mineral was 

 an intimate mixture of fine crystals of blende and galena, the blende 

 being the transparent particles and the galena the opaque. Although 

 both these minerals possess a certain degree of metallic lustre, galena 

 is one of the most perfectly opaque substances known, whilst blende 

 in very thin layers is perfectly transparent. 



* Ann. and Mag. Nat. Hist., xvi. (1885) p. 145. 



