84(3 SEC. 16. GEOLOGY AND MINING. 



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 Rocks. 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 

 fine particles are sorted or not into thin or thicker bands according as the 

 screw is moved in an irregular or regular manner, just as occurs when a 

 current moves with varying velocity. The arrangement of the materials thus 

 produced is in all respects similar to what is so often seen in certain beds of 

 stratified rocks. 



3298. Microscopical Sections of Shells and Bocks. 



H. C. Sorby. 



These are prepared by grinding down one side to a perfectly flat and 

 smooth surface, which is then fixed to a glass with Canada balsam. After 

 this, the other side is ground down, first with emery and at last on a very 

 hard piece of water of Ayr stone, so as to leave a thin portion of the rock in 

 a perfectly undisturbed condition attached to the glass, with a smooth and 

 almost polished surface. According to the nature of the rock, the thickness 

 of such sections should vary from about y^- to 10 1 00 th of an inch. Thin 

 glass is then mounted over the whole with Canada balsam. As examples of 

 rocks presenting special difficulties, attention may be drawn to the sections 

 of soft chalk, and of slate and mica schist, cut perpendicularly to the cleavage 

 or foliation. 



3299. Lithographed Plates, illustrating the Microscop- 

 ical Structure of Limestones. H, C. Sorby. 



These show structure due to larger or smaller fragments of organic bodies 

 or grains of sand, and to the more or less complete development of crystals 

 formed either during or after the deposition of rock. 



3300. Working Model, illustrating the Movement of 



Waves in forming ripple marks. H. C. Sorby. 



The model shows the movement of the particles at the surface and at the 

 bottom of a wave which has advanced from deep to shallow water, so as to 

 give rise to ripple marks. At the surface the water moves nearly in circles, 

 which in the model is represented by the white discs attached to arms con- 

 nected with each alternate wheel. At the bottom the water moves forwards 

 and backwards, drifting the sand in the line of the movement of the wave 

 wheii its crest is passing, and backwards when the trough is passing. In the 

 model this movement is produced by the action of small eccentric wheels on 

 pieces of brass fixed at one end and carrying white discs at the other. 



3300a. Specimens relating to the exhibitor's researches on 

 synthetical and experimental Geology. 



1. Minerals formed at about 400 by superheated water (1860). 



2. Product of the reactions of vapours. 



