of the Cumbrian Mountains. 67 



between the primary stratified groups and the deposits superior to the chalk. 

 In some instances secondary rocks have put on the exact appearance of 

 the primary : but is there a tertiary deposit in any country yet examined, 

 which could for a moment be confounded with the greywacke systems of 

 Cumberland or Devonshire? Through large regions of the earth, the epoch of 

 many deposits is not less defined by the mineral structure of the rocks than by 

 their organic contents. Accurate mineralogical distinctions, and an order of 

 superposition determined by natural sections, must form the first foundations of 

 the geology of every country. Organic remains often help us to unite together 

 disconnected base Hues. They also enable us to subdivide the successive de- 

 posits of one epoch, in cases where all other means fail ; and in speculating 

 on the former conditions of the earth they are invaluable : but they can in 

 no instance supersede the necessity of studying in detail the structure and 

 superposition of the great mineral masses composing the crust of the globe. 



(j. In the Cumbrian system, the elevation of the old slate rocks appears to 

 have been produced by the protrusion of mountain masses of granite and 

 syenite. In the higher parts of North Wales, the strata are thrown into vast 

 undulations ranging parallel to the principal mountain chain ; but we find no 

 obvious centre of dislocation. This fact however throws no difficulty in the 

 way of the igneous hypothesis of elevation ; as we know that the most violent 

 volcanic action is often felt at a great distance from the focus of eruption. 

 The parallelism of the undulations seems however to prove, that they were 

 not produced by different shocks of earthquakes succeeding each other after 

 long intervals of time. 



As the earth has apparently diminished in temperature, we have a right to 

 look for some indication of a contraction of its dimensions. May not some of 

 the great parallel corrugations of the older systems of strata have been pro- 

 duced by such a partial contraction ? 



7. In stating that volcanic action could not be brought under the laws of 

 any constant force, I merely made a reference to the irregular and undefined 

 nature of its effects. The degradation of the solid portions of the earth by 

 aqueous action may in a certain sense be considered constant ; because there 

 is a demonstrable provision in the laws of nature for the perpetual circulation 

 of water over the surface of the globe. But who can point out^ compatibly 

 with the known laws of chemical action, a provision for the eternal and uni- 

 form circulation of volcanic fire through all parts of the crust of the globe — 

 to go its rounds with regularity, and, during given periods of time, neither to 

 increase or diminish in intensity ? If such a supposition be started, it can 

 only be regarded as a mere gratuitous and most complicated hypothesis ; and 

 ought not ever to be assumed as the basis of a priori reasoning. 



K 2 



