THE ANALYSES. 



27 



AVERAGES OF THE GROUPS. 

 TABLE 13. Igneous rocks. 



The general averages bring out the fact that, while rocks of each group 

 may vary considerably among themselves, each group as a whole fits into 

 a logical place in relation to the other groups. The established order 

 appears to be, most gas from those rocks which contain the greatest pro- 

 portion of ferromagnesian minerals. Though much influenced by other 

 conditions, such as relative age and nature of the igneous mass, the general 

 deduction may be made that the volume of gas obtained from rocks 

 varies, in a rough way, in proportion to the percentage of ferromagnesian 

 minerals present. Diabases, basalts, and basic schists take first rank in 

 the quantity of gas evolved. Next to them appear diorites and gabbros 

 which are also near the basic end, but formed under different conditions. 

 Andesites are out of their place in this list, as they take precedence over 

 granites in the proportion of ferromagnesian minerals, but these andesites 

 were all either of Tertiary or Recent age, whereas most of the granites came 

 from Pre-Cambrian formations, and, as the next table will show, ancient 

 igneous rocks yield more gas than modern ones. The rhyolites, which com- 

 bine a scarcity of basic minerals with Tertiary age, foot the list. 



It is to be noted that the rank of a type of rock on the basis of an 

 individual gas does not in all cases correspond to its rank for some other 

 gas, or in respect to total volumes. The andesites tested gave more carbon 

 dioxide than either the granites or the syenites, though both of these types 

 greatly surpassed the andesites in the matter of hydrogen. But this in- 

 volves another factor: in deep-seated rocks, hydrogen and carbon dioxide 

 are of about equal importance; in surface flows, carbon dioxide predomi- 

 nates. Though carbon monoxide and methane are somewhat variable, 

 the minor gases generally increase or decrease with the total volumes. 



TABLE 14. Rocks of sedimentary origin. 



Among sedimentary rocks, sandstones and quartzites yield less gas 

 than shales, while the metamorphic group, comprising both altered shales 

 and sandstones, together with modified limestones, take an intermediate 

 position, though they surpass shales in hydrogen and the sulphur gases. 



