THE STRUCTURE AND CLASSIFICATION OF ROCKS. 



625 



arc said to be so used at present in some remote parts of Russia. These lamina?, however, 

 often give an appearance of fissility to a rock which is deceptive, the layers separating 

 with great difficulty. Though in general parallel to the greater lines of stratification, the 

 lamination is frequently inclined and wavy. Fig. 1 shows several varieties. Occasionally 

 the lamination is highly undulating, tortuous, and angular. Fig. 2 is an instance of very 

 contorted lamination, from a loose block of gneiss, two or three feet thick, in Colebrook in 



Connecticut. Fig. 3 is a specimen of zigzag or angular lamination, from the coal. In a 

 bed of diluvial clay, in Decrfield in Massachusetts, disturbed layers of clay appear in 

 tervening between others, which are horizontal and undisturbed, as in Fig. 4. The dis 

 turbed lamina? here must have received their flexure subsequent to deposition by some 

 operation acting upon them alone. The laminar structure is almost entirely confined to 

 rocks which have been originally produced by deposition in water, and its varieties are 

 the result of the different circumstances under which deposition has taken place, together 

 with pressure acting upon the mass while yet in a plastic state. Thus the horizontal 

 parallel lamina? result from quiet deposition in still waters upon a level surface. The waved 

 laminae bear witness to deposition in shallow water, and show what is commonly called 

 the ripple-mark, which we may every day observe upon our own shores, for, when the 

 tide has ebbed, there is the impression of its retreating ripples on the sand. The in 

 clined lamina? may have been caused by deposition on a variously inclined shore : and the 

 highly contorted and zigzag laminations have probably resulted from lateral or vertical 

 pressure operating after deposition, before the strata had become indurated. It will be 

 seen that the laminar structure of a stratum bears the same relation to it, as its own 

 stratification does to a whole series of beds. 



The external structureof rocks may be conveniently considered under thetwo grandclasses, 

 of the stratified and unstratified, which include every kind of formation. The unstratified 

 rocks are those of igneous origin, as granite, porphyry, and serpentine, which show no 



regular divisional structure resembling a series of beds, but occur in enormous masses, only 

 broken by irregular fissures in different directions. Their aspect to the eye is variously 



