THE ANALYSES. 



29 



Age appears to make less difference in the gas evolved from sedimentary 

 or meta-sedimentary rocks than it does in the case of igneous rocks. All 

 of the Proterozoic specimens were of metamorphic types, while only one 

 of the Paleozoic sediments had been metamorphosed. The Mesozoic repre- 

 sentative was a Jurassic shale altered by an intrusive. The unusual amount 

 of sulphur gas in the Proterozoic list is due to two weathered rocks which 

 contained iron sulphate. However, even with these omitted, the hydrogen 

 sulphide is abnormally high in the rocks of this age. One of the Paleozoic 

 shales was so calcareous as to yield 9.28 volumes of carbon dioxide, which 

 accounts for the large quantity of this gas. The two bituminous shales 

 (analyses 41 and 42) are not included in these averages, since their exces- 

 sive volume of gas from organic sources would so influence the figures as 

 to disguise some of the characteristics of the other rocks. 



ANALYSES CLASSIFIED BY THE GRANULARITY OF THE ROCKS. 



TABLE 18. Igneous rocks. 



From this table it would appear that the fine-grained rocks give off 

 more gas than those of coarser granularity. One of the reasons for this 

 difference probably lies in the fact that metasomatic changes are favored 

 in fine-grained rocks, whose crystals, being smaller, afford more numerous 

 junction-planes between the crystals, through which solutions more readily 

 traverse the rock than in the coarse-grained varieties. Among other 

 changes, hydration and carbonation should alter fine-grained rocks more 

 effectively than coarse-grained ones. 



Fineness of grain in igneous rocks usually means that the lava cooled 

 rapidly, and this would hinder the escape of the inclosed gas. But in the 

 process of slow crystallization, such as produces large crystals and coarse 

 texture, much more of the gas would be likely to be crowded out of the 

 growing crystals. However, as a general rule, fine-grained igneous rocks 

 are surface flows, while coarse-texture types were formed at some depth 

 below the surface, and hence a larger proportion of whatever gas was 

 expelled from the rapidly cooling lavas would be more likely to escape 

 altogether than would be the case with the gas which was excluded from 

 growing crystals in deeper horizons, as in bathylithic intrusions, where final 

 escape was difficult. In this problem of granularity, as in the matter of age, 

 the quantities of gas evolved are probably determined by a combination of 

 complex factors rather than by any single cause. 



RESULTS AT DIFFERENT TEMPERATURES. 



The different gases are not all expelled from rock material at the same 

 temperature, nor are they evolved at the same rate. In general, hydro- 

 gen sulphide and carbon dioxide are not only the first gases to appear, but 



