1897. NOTES AND COMMENTS. 79 



We incline to think that any investigation which treats rock- 

 masses as homogeneous substances must of necessity be unsatisfactory. 

 The commonly accepted theory of the cleavage-structure has for its 

 starting-point the heterogeneous structure of the rock affected. Differ- 

 ing as to postulates, Mr. Becker's theory also differs in its conclusions 

 from the commonly received one. He finds that the structures 

 induced must stand at acute angles to the causative force, instead of 

 being perpendicular to it. This is very difficult to reconcile with the 

 evidence presented by rock-masses in the field. It is also inconsistent 

 with the evidence of the deformed concretions, fossils and the like, in 

 numerous slates, which give us an actual picture of the strain-ellipsoid ; 

 in these cases the cleavage plane is always perpendicular to the least 

 axis of the ellipsoid. While the paper is in itself of much interest, it 

 may be regretted that the author has not made a fuller comparison 

 between his theoretical conclusions and the facts of observation. 



The Classification of Precambrian Rocks. 



There has always been a tendency for men to classify natural 

 facts, not according to knowledge, but according to ignorance, just as 

 we even now continue to define the genera of living beings by the 

 breaks in the chain, or the periods of geology by the gaps in the record. 

 It is as a consequence of this human frailty that the history of geolo- 

 gical science shows us a continued attempt to separate certain of the 

 oldest rocks from their successors on the ground of the absence from 

 them of fossils, and to label them Azoic or some equivalent name. As 

 our knowledge grows the limits of this primitive group of rocks 

 become more restricted ; so that from having included all strata below 

 the Devonian, it now includes only comparatively few of the ancient 

 crystalline rocks. But the want of logic in this division is shown not 

 only by continual fresh discoveries of fossils, but by the assumption, 

 nowadays universal, that the oldest animals known to us were pre- 

 ceded by a long array of ancestors. Acting, no doubt, on this con- 

 viction, at a recent meeting of the Geological Society of Stockholm, 

 Dr. J. j. Sederholm, head of the Geological Survey of Finland, 

 pointed out how difficult it was to draw a true line of demarcation 

 between the strictly preorganic or azoic rocks and those that possibly 

 were once fossiliferous. He would designate the whole basal complex 

 as " Archaean," without reference to contained organisms, separating 

 from the succeeding " Archaeozoic" on the ground of its having 

 been affected by certain earth-movements of earlier date than the 

 deposition of the latter rocks. The Archaeozoic rocks again, 

 for the Scandinavian region at least, Dr. Sederholm would divide 

 into the older, " Jatulic," which have been involved in certain great 

 mountain-folds, and the younger, "Jotnic," which are later than 

 these folds. 



