XXV. — Remarks on the Structure of large Mineral Masses, and espe- 

 cialhj on the Chemical Changes produced in the Aggregation of 

 Stratified Rocks during different Periods after their Deposition. 



By the Rev. ADAM SEDGWICK, F.G.S. F.R.S. &c. 



(WOODWARDIAN PROFESSOR IN THE UNIVERSITY OF CAMBRIDGE.) 



[Read March 11, 1835.*] 



§ 1. Introductorij Remarks, ^c. 



All solid mineral masses must have undergone some change since the time 

 of their first production. Beds of secondary limestone and sandstone did not 

 drop to the bottom of the sea, layer upon layer, in a solid form ; and it is 

 equally certain (thoug-h not equally obvious) that large unstratified crystalline 

 masses were not created as we now find them. No one supposes that co- 

 lumnar basalt was originally built up of solid parallel jointed pillars, or that 

 the structure of a granitoid rock was effected by a mere fortuitous concourse 

 of the crystalline parts. We believe that these phagnomena are the necessary 

 consequences of a certain anterior condition of the materials we examine. 

 Sometimes, indeed, we can imitate these conditions, and then (as the laws of 

 nature are unchangeable) we can do over again that which has been done a 

 thousand times before in the laboratory of nature. 



Many large mineral masses appear to have been once in a state of igneous 

 fusion. Such masses, in passing from a fluid (or semi-fluid) to a solid state, 

 necessarily put on a form more or less crystalline. The crystalline form is 

 therefore the first and inevitable change. But there is another effect, arising 

 out of such changes, of great geological importance. The mass which has 

 changed its temperature, and become solid, has also changed its dimensions. 

 Contraction must produce tension on the whole mass ; and this tension, acting 

 mechanically, will in many instances produce joints and fissures, and some- 

 times contortions : these effects will be of greater or less regularity according 

 to the conditions of each particular case. 



* The Council has thought it advisable to publish this memoir before its turn, because they 

 consider it to be introductory to other papers by tlie same author, some of which are already 

 printed, and will appear in the fourth volume, illustrative of the origin and structure of the older 

 stratified rocks. 



VOL, HI. SECOND SERIES. 3 O 



