I. GEOLOGY. 713 



collection comprises about 20 specimens, average dimensions 1O 

 centimetres by 15. 



Six chromolithographs of drawings of minerals and rocks as 

 seen through the microscope. A. Renard, Louvain. 



3293. Specimens illustrating the production by Compression 

 of Natural and Artificial Slaty Cleavage. //. C. Sorby. 



Specimens of slate rocks, showing, by various facts, that they have been 

 greatly compressed in a line perpendicular to the cleavage. 



Pipe-clay mixed with portions of blue paper, and also with iron scales, 

 being the results of the first experiments made to show that a structure like 

 that which causes the cleavage in slates can be artificially produced by 

 pressure. 



Artificial cleavage in compressed flaky graphite, being as perfect as that 

 in any slate rock. 



3294. Specimens illustrating the Metamorphic Origin of 

 Mica Schist, and the difference between stratification, foliation,, 

 and cleavage foliation. H. C. Sorby. 



" Ripple drift" in slate rocks in which the cleavage cuts the stratification at 

 a considerable angle. 



" Ripple drift " in contorted ^and highly metamorphosed mica schist, thus 

 proving the original stratified nature of the rock. 



Mica schist with foliation in the plane of stratification, being rock metamor- 

 phosed before being compressed. 



Mica schist with foliation in the plane of cleavage, developed by compres- 

 sion, before the work was metamorphosed. 



3295. Microscopical Photographs of sections of iron and 

 steel. H. C. Sorby. 



The above were photographed by means of strong surface illumination, 

 showing structures due to the arrangement of crystals of iron combined with 

 a varying amount of carbon, of portions of slag, and of crystalline plates of 

 graphite. Note the contrast between the structure of cast iron, cast steel, 

 and meteoric iron, although all have solidified from fusion. 



3296. Microscopical Sections of iron and steel. 



H. C. Sorby. 



The above were prepared by very carefully grinding down and polishing the 

 surface so as to avoid all such burnishing action as would alter the form or 

 structure of the ultimate crystalline particles. The whole section is then 

 placed in very dilute nitric acid, and carefully examined in water under the 

 microscope, time after time, until the irregular action of the acid on the 

 different constituents has advanced so far as to show the general structure to 

 the greatest advantage. The surface is then well washed, dried, and gently 

 wiped, and finally protected by a thin glass cover cemented down by Canada 

 balsam. 



3297. Working Model illustrating the Formation of 

 "False Bedding" in Stratified Bocks. H. C. Sorby. 



The drifting action of the current of water is represented by the screw, 

 which carries forward the sand until it falls down and accumulates on the 

 slope at the angle of rest. The larger fragments roll to the bottom, and the 



