PRELIMINARY HANDBOOK OF THE DEPARTMENT OF GEOLOGY. 25 



exhibition purposes ; (2) joints in both stratified and massive rocks ; 

 (3) inclination, strike, and dip of rocks (shown only by models and 

 photographs) ; (4) curvature, cleavage, distortion and dislocation, and 

 other modifications of the primary arrangement of the earth's crust ; (5) 

 igneous rocks as a part of the structure of the earth ; and (6) veins and 

 vein materials. 



Many objects are here displayed of precisely the same nature as under 

 dynamical geology, in the one case illustrating the methods by which 

 certain structures are brought about and in the other the structures 

 themselves. The text- book arrangement has not here in all cases been 

 strictly adhered to, as not being quite adapted to our purpose. 



(1) Stratification and its accompaniments. — (a) Forms of bedding: This 

 department of the exhibit is at present sadly in need of suitable mate- 

 rial. The few specimens at present on hand are small and poor, and 

 need not therefore be further noted here. 



(b) Surface markings : These are shown in their several forms as 

 below : 



Wave and ripple marks are shown in the two large slabs of Fotsdam 

 quartzite from Keeseville, New York (38758), and the Devonian sand- 

 stone from Pike County, Pennsylvania (27014). Several smaller slabs 

 from various localities are shown in the cases. To appreciate these it 

 must be remembered that the rocks were lain down in the form of fine 

 sand in the shallow water of an ancient sea or ocean, and which by its 

 oscillatory movement formed in the sand the ripples in the same manner 

 as they may to-day be seen forming on almost any sandy lake or sea 

 shore (Geikie, p. 470). Through the gradual sinking of the earth's 

 crust the markings once formed became covered by other sand and thus 

 preserved until thoroughly solidified, raised above sea level, and made 

 available to the quarrier, all the slabs shown being obtained in the 

 ordinary process of quarrying stone for building purposes. " Ripple 

 marks are often made by the waves over the finer beach sands where 

 they are low and partly sheltered, and also over mud flats. The flow- 

 ing water pushes up the sand into a ridgelet as high as the force of the 

 wave can make and then plunges over the little elevation and begins 

 another, and thus the succession is produced. The height and breadth 

 of the intervening space will depend on the force and velocity of the 

 flowing water and the ease with which the sand or mud is moved. Rip- 

 ple marks may be made by the vibration of waves at depths of 300 or 

 500 feet."— (Dana.) 



Mud cracks and rain prints : Like the ripple marks the mud cracks 

 denote shallow water deposits. Laid down as mud, the beds while still 

 soft were exposed by the receding water and dried, cracking irregularly 

 just as the tine clayey mud on the bottom of shallow pools may in any 

 dry season be observed to do. Subsequently the water rose once more 

 and washed fresh sand into the crevices formed. The elevated ridges 

 shown on the large slab of Medina stone from Knowlesville, New York 



