Composition i and Analogies of Rocks, 207 



chain through the varieties of this rock which connects it with 

 the secondaiy sandstones as perfectly as quartz rock is. The 

 causes for the evanescence of this analogy consists in the pos- 

 terior influence of heat. In the same manner, the action of heat 

 has converted shale into hornblende ; and thus, in the frequent al- 

 ternations of gneiss and hornblende schist, we have an exact coun- 

 terpart of that alternation so common between the oldest of the 

 secondary sandstones and its concomitant shale. I need not dwell 

 longer on a question which, interesting as it may be esteemed, is 

 too deftcient in accuracy of evidence to be a very satisfactory sub- 

 ject of discussion, 



Although it has thus been inculcated that all the stratified rocks 

 which are not the produce of animals, have ultimately been de- 

 rived from former rocks, and probably in a series of succession 

 the limits of which we cannot pretend to conjecture, it is still 

 proper to remark, that there is a progressive change of character 

 as we retreat. The limestones, it has been particularly shown, 

 become more rare, but the argillaceous substances diminish also ; 

 so that at length, in arriving at that antiquity which, to our ob- 

 servation, is the highest, siliceous rocks predominate in a great 

 degree. Thus a short-sighted philosophy might arrive at a con- 

 clusion the reverse of that formerly suggested with respect to the 

 increase of calcareous strata, and imagine an universe once as 

 incapable of maintaining vegetables, as it has, to all appearance, 

 been limited in the numbers and nature of its animals ; a desert 

 of rocks and sand. But tliis conclusion is not justified when we 

 take a general view of all the phenomena that geology presents. 

 That it has been drawn, has arisen either from false theories or 

 partial views. If the siliceous substances predominate in the 

 more ancient parts of the series, it must be remembered that these 

 are but the remains of rocks, of which the greater part has dis- 

 appeared to form the present secondary strata ; nor, in the revo- 

 lutions of ages, can we decide on what has vanished, and what 

 the state of the more ancient surface was. That it furnished a 

 vegetable creation, and that also to a great extent, is evinced by 

 the phenomena of coal strata, and by the enormous masses of 



