ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 297 



Although this language is somewhat indefinite, still the idea 

 embodied in the igneous theory is shadowed forth in it, and on 

 the whole this quotation may be considered as the text of the 

 present essay. It is I believe possible to maintain, with every 

 appearance of reason, that the Primitive Gneiss formation con- 

 stitutes the first solidified crust of the originally fused globe, 

 and that the crystalline and sub-crystalline rocks of the Primitive 

 Slate formation are the products of a peculiar transition period, 

 during which aqueous fluids gradually accumulated on the surface, 

 and the latter attained a temperature approaching somewhat to 

 that of the present day. 



In attempting to show that this proposition is supported by 

 geological evidence, I shall confine myself principally to arranging 

 and elaborating the facts and arguments in support of it, which 

 I have found scattered through a considerable number of geo- 

 logical papers and manuals. I shall also, in order to state the 

 case with full force, be obliged to insert prefatorily much of what 

 may be considered as mere elementary facts in physical geography 

 and geology. I shall first refer to the evidences which we 

 possess regarding the internal heat of our planet and its density, 

 deducing from them certain conclusions as to the present con- 

 dition of the interior of the earth. In doing so, I shall allude 

 to the nature of certain volcanic products ; and then continuing 

 the considerations of the constitution and mode of occurrence 

 of igneous rocks, I shall search back through the various eruptive 

 formations for evidences of the nature of the igneous action 

 which has taken place in former periods of the earth's history, 

 and ultimately arrive at the consideration of the theory of the 

 earth's original state of igneous fluidity. This theory, univer- 

 sally admitted by geologists, will then afford us a firm starting 

 point for some speculations as to the process of the first solid- 

 ification of the earth's crust, and the origin of gneissoid rocks. 

 Pursuing the subject further, I shall endeavour to shew that 

 the peculiar rocks of the Primitive Slate formation are also pro- 

 ducts of the action of the first condensed fluids on the heated 

 crust of the earth. There are few theories whereon such a 

 unanimity of opinion exists among geologists, as that of the 

 originally fused condition of our planet, and few formations 

 regarding the oiigin of which more uncertainty prevails than 

 that of the primitive formations. If therefore it can be shewn 

 to be probable that these primitive formations have merely 



