﻿J. D. Dana — General terms applied to Metamorphism. 69 



Fig. 7 shows how, with a compound microscope, having its 

 fine adjustment screw furnished with a micrometer-head, we 

 can measure with a well of small diameter the radii of curva- 

 ture of very small lenses. A, B, is the plate with the well of 

 small diameter. This plate rests on another plate, C, D, with a 

 fine scratch on its upper surface, in the center of the bottom of 

 the well. We first place a piece of thin glass, with a fine line 

 on it, on the top of A, B, with the surface carrying the line 



ffi 



resting on the top of A, B, and focus on this line and then take 

 the reading of the screw-head. This plate is then removed 

 and we focus on the line at the bottom of the well, and again 

 take the reading. The difference of readings gives us the 

 depth of the well. Then the lens, which has a fine mark on it 

 where its principal axis cuts its surface, is placed on the glass 

 plate C, D, and the microscope is focussed on the fine mark on 

 the top of the lens, and the reading of the screw taken. The 

 lens is now removed and the microscope is focussed on the fine 

 line on C, D, and the reading of screw is taken. The differ- 

 ence of these readings gives the thickness of the lens. The 

 lens is now placed, as shown in figure 7, covering the top of 

 the well. The line on the top of the lens is again focussed and 

 the reading taken. Then the lens is removed and the micro- 

 scope is focussed on the line at the bottom of the well, and the 

 screw-head reading taken. Call this measure t' ; the depth of 

 the well t, aud the thickness of the lens I. Then (t+l) — t f = 

 the depth of the section of the lens below the top of well ; and 

 as the radius of well is known, we can compute the radius of 

 the lens. 



Aet. VIII. — On some general terms applied to Metamorphism, and 

 to the Porphyritic Structure of Rocks ; by James D. Dana. 



A. Metamoephism.* 



The ordinary methods of metamorphic change recognized 

 in geology are the following: 



(1) The development of a crystalline condition in the original 

 materials ; recently shown by Tornebohm. Sorby, Irving and 



* The terms applied to metamorphism are considered in the Presidential Ad- 

 dress of Professor T. G. Bonney before -the Geological Society of London, in 

 February last ; but the copy of the address did not reach the author until June 

 22, when this paper was just going to press. It does not call for any change in 

 the remarks here offered. 



