PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 15 



of wax of different colors and consistency were first laid horizontally 

 on a board and weighted down by means of bird shot placed on the top. 

 Pressure was then applied from the direction of the two ends (the right 

 and the left) when the mass passed through the various stages shown 

 in the photographs and finally assumed the shape in which it is now 

 exhibited. 



Although a mass of rock may seem to us firm and unyieldiug, it is 

 nevertheless susceptible of undergoing just such a crumpling and fold- 

 ing as has this wax model through the long continued compressive and 

 shearing force which from time to time has manifested itself in various 

 parts of the earth's crust. A few examples of such folds and crumplings 

 or contortions are here shown, and a larger, more varied series may be 

 found under the head of structural geology. Attention may be called 

 to the contorted gneisses from Brandon, Vermont (39818) ; Sherburne 

 Falls, Massachusetts (38124); Stony Point, New York (38746), and 

 Norway (70422). 



Such a force does not always,- result, however, in the production of 

 folds, but as is now well known may be productive of a foliated or 

 schistose structure. 



Interesting examples of such results are shown in a series of specimens 

 from Slatington, Pennsylvania (70104-70107 and 702G6-70267). To 

 fully appreciate this exhibit it must be remembered that the slates were 

 originally formed as fine sediments laid down on a sea bottom, and that 

 the cleavage whereby they split up readily into thin sheets is due 

 wholly to subsequent pressure (see Geikie, p. 288) and is in no way con- 

 nected with the bedding which may cross it at any and all angles. In 

 the specimens the bedding is indicated by the dark bands and the 

 cleavage is plainly shown cutting across it. 



In certain of the samples the material of these dark bands seems to 

 have lent itself less readily to the compressive force, and may be ob- 

 served to have puckered (specimens 70104 and 70105) or even to have 

 broken and faulted repeatedly, as in the block No. 702G6. 



All rocks are not equally tough or elastic, and as may readily be 

 imagined do not always show similar effects under the action of the 

 same forces. Many brittle rocks are shattered into innumerable frag 

 ments under a shock or such pressure as would be productive of far less 

 striking results on tougher materials. This shattering effect is shown 

 in the collections by the polished slabs of limestone breccia from Algeria 

 (G9574). The fragments in this case have been reunited by the cement- 

 ing action of infiltrating water carrying lime and iron oxides in solution 

 and form thus our richest and most prized marbles. 



Other interesting indications of great pressure and strain are shown 

 in a series of indented and crushed pebbles. The large quartz pebble 

 from Silesia (13048) is peculiarly interesting, having been broken re- 

 peatedly in two directions and one portion pushed over the other for a 

 distance of a centimetre or more. Yet the parts have so firmly reunited 



