374 J. D. Dana — On displacement through intrusion. 



But along the Lexington branch of the same road, the beds 

 from the Cambrian to the Trenton are cut by the road and the 

 river (the channel of which it follows) in such a way as to ex- 

 pose in a striking manner the numerous faultings and foldings 

 to which this portion of the Great Valley has been subjected at 

 some remote period of its history. A profile section from the 

 crest of the Blue Eidge through Lexington and cutting the 

 House Mountains, may be seen in this Journal, vol. xviii, 1879, 

 p. 19. It is designed to show in a graphic way a variety of 

 structures not often found within the same compass. 



Our sketch has now brought us to the top of what is gen- 

 erally regarded as the Lower Silurian Age. We have dwelt 

 upon this portion of the geology of Virginia at some length ; 

 first, because the formations of Which we have been treating 

 are remarkably well and widely developed in this State ; and 

 secondly, because we desire to give our fellow-students the 

 benefit of a concise, and, as far as space and time will permit, a 

 systematic view of the field which is still open for additional 

 investigations. The higher formations we shall discuss here- 

 after. 



"Washington and Lee University, Va., September, 1885. 

 [To be continued.] 



Art. XL VIII. — On displacement through intrusion; by James 



D. Daista. 



The wedging action of intruding material may act either in 

 fissures and crevices, or among the grains or constituent par- 

 ticles at or near the surface of a rock ; and the displacement 

 that results may consist in the opening and widening of fissures, 

 in the separation of masses, in the faulting of the separated frag- 

 ments, and in rock disintegration. The methods are either 

 organic, molar or molecular. The first includes, (1) the 

 intrusion of vegetable growths from microscopic plants to the 

 roots and stems of trees; the second (2) the intrusion of melted 

 rock into fissures or between layers; (3) the intrusion of gas 

 under pressure, or when suddenly developed ; and the third com- 

 prises (4) the freezing of intruded water, (5) chemical deposition 

 from a solution of outside origin ; (6) chemical deposition or -com- 

 position, arising from changes within the rock. The minutest 

 grain of a new mineral made or deposited within the rock has a 

 displacing and disruptive action or tendency. 



The production of a pseudo-breccia f rom quartzyte, described 

 by me in this Journal for last November and January (xxviii, 

 451, xxix, 57), is an example of displacement by one of the 



