344 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



" dolerite, anamesite, and basalt. It snows quite similar amyg- 

 " daioidal forms to those of the latter rocks. It is a massive 

 " rock sometimes with columnar development, a completely non- 

 " fossiliferous rock like basalt. All these coincidences, from a 

 " lithological point of view alone, appear completely to justify the 

 " view, that the melaphyrs like the basalts must be numbered 

 •' among pyrogenous rocks. In the felsite-porphyries, it is true that 

 " common orthoclase takes the place of the glassy felspar of the 

 * : trachytes, still the difference betwixt these two minerals must 

 " be looked upon as trifling, especially when it is remembered, that 

 " most orthoclases contain some soda besides the potash. More- 

 " over, the remaining constituents, albite, oligoclase, mica, and 

 " quartz are common to the trachytic-porphyries and to the 

 " andesites, as well as to the felsitic-porphyries, while the labra- 

 " do rite brings certain porphy rites in very close relationship to the 

 " melaphyrs, from which they are sometimes almost undistinguish- 

 u able. The unprejudiced enquirer will therefore surely without 

 " hesitation regard the felsitic porphyries as rocks quite analo- 

 " gous to the trachytic porphyries, with which they also 

 " correspond in many other properties. There are also other 

 " rocks, regarding the origin of which we must come to similar 

 " conclusions. The diabases consist essentially of oligoclase or 

 *' labradorite and pyroxene; the diorites of albite, hornblende, 

 " and quartz ; both classes therefore of exactly the same minerals 

 " as we observe occurring in lavas, basalts and trachytes. In 

 " mineralogical and chemical respects therefore, no objection can 

 " be taken to the supposition that they have been formed in a 

 " manner exactly similar to these latter rocks. When we add to 

 " this that these greenstones are always completelynon-fossiliferous, 

 " generally massive and supplied with structures and forms of 

 " deposition quite similar to those of the basalts and lavas, the 

 " above supposition would appear to be in every respect justifi- 

 " able." With regard to the chemical composition of these rocks 

 we find, that if we take the analysis of three hornblendic por- 

 phyries, and of a similar number of felsitic porphyries, as given 

 in Bishop's Chemical and Physical Geology, the contents of silica 

 of these will range from 11.9 to 59.81 and average 61.11 per 

 cent. If we further take the analysis of greenstones and mela- 

 phyres contained in the same work, we find their percentage of 

 silica to range from 55.29 to 42.72, and average 50.9. 



The porphyries seem to have been formed principally during 

 the Carboniferous and Permian periods. They often occur in the 



