66 -/. Croll — Evidence of former Glacial Periods. 



orthopinacoid, which affords surfaces suitable for measurement, 

 but the other planes in the orthodome zone are striated parallel 

 to the macrodiagonal axis, and the angles they yield are less 

 satisfactory. The crystals when newly prepared are very 

 bright and transparent, but soon lose their luster on exposure 

 to the air. 



For fundamental angles the following were accepted : 



oo'\ III - 111=47° 30', co, 001-111=76° 0', ao", 100- 111 = 75° 1\'. 



The axial ratio obtained is as follows, that of Zepharovich 

 being added for sake of comparison : 



_ i 



a: b: c = 2-8426 : 1 : 2"9494, /3=76° 31'. 



I 

 a : b : c=2"8459 : 1 : 2'9696, /3=77° 23' Zeph. 



The following table gives a comparison between calculated 

 and measured angles : 



Calculated. 



Ill /.111 = 47° 30' 



001-111 = 76 0- 



100- 111 = 75 7 30" 



100- 111 = 67 14 13 



100- 101 = 37 25 28 



001 - 101 = 39 5 33 



111 - 111 = 37 38 17 



001 - 111 = 68 8 29 



100-211 = 52 41 8 



001-302 = 68 10 14 



001-100= 76 31 1 



001 -011 = 70 46 43 



100- 110 = 70 6 44 



100-101 = 37 25 28 



Art. VIII. — On prevailing misconceptio?is regarding the 

 Evidence which we ought to expect of former Glacial 

 Periods /* by James Croll, LL.D., F.R.S. 



Within the whole range of geological science there is per- 

 haps not a point on which a greater amount of misapprehen- 

 sion prevails than in regard to the evidence which we ought to 

 expect of former Glacial periods. The imperfection of geo- 

 logical records is far greater than is generally believed : so 

 great, indeed, that the mere absence of direct geological 

 evidence can hardly be regarded as sufficient proof that the 

 conclusions derived from astronomical and physical considera- 

 tions regarding former ice-periods are improbable. ISTor is this 



* From the Quarterly Journal of the Geological Society of London for May 

 1889, by request of the Author. 



Measured. 





Limits. 





*47 



30' 



47° 



19£ 47° 



56. 



*76 







75 



to 78 





*75 



7 30" 



75 



to 75 



36" 



67 



13 



65 



53 to 68 



23 



38 



approx. 









39 



35 



40 



39 to 40 



38' 



37 



32 



36 



35 to 36 



39' 







52 



26 * 53 



8 



52 



47 









67 



approx. 









