70 PASCOE: GEOLOGICAL NOTES ON MESOPOTAMIA. 



Conglomerates. 



It requires experience to be able to distinguish the conglomerate 

 • of the Conglomerate stage (Nasaz zone) from Pleistocene or Recent 

 conglomerates and gravels, especially in the neighbourhood of large 

 rivers. As the pebbles of the younger are precisely similar to. and 

 probably nearly all derived from, the older conglomerate, there is no 

 distinction in character between them, and position and dip are the 

 most important criteria. Some of the thinner layers in the Nasaz 

 zone are scarcely more than stringers of loose gravel, while the 

 vounger Pleistocene deposit, on the other hand, may be a very hard 

 tough conglomerate, such as that on the cliffs above Qal'at-al-Bint. 

 In widely isolated exposures it is sometimes impossible to determine 

 the age of a more or less horizontal conglomerate. 



Formation of Hydrogen Sulphide. 



In Report No. 2 it was suggested that the hydrogen sulphide 

 associated with the tarry oil and bitumen seepages, might be the 

 outcome of some chemical action between crude petroleum and sul- 

 phates, more espaciaily between petroleum and gypsum, I he 

 sulphuric acid radical of the latter being thereby reduced to 

 hydrogen sulphide. Subsequent experience abundantly supports 

 this view which is not a new one, but has been brought forward 

 by others to explain similar phenomena in other countries. 

 Beyerinck and also Saltet have shown that oxygenated sulphur 

 compounds are reduced by anaerobic bacteria to hydrogen sulphide 

 in the presence of dead organic matter. It is very unlikely, how- 

 ever, that such organisms can exist at any appreciable depth below 

 the surface, but experiments by Kharitschoff show that similar reduc- 

 tion of sulphates in solution— gypsum and to a still greater extent 

 magnesium sulphate — are brought about slowly, especially under 

 high pressure and temperature, by hydrocarbons. Hofer gives equa- 

 tions representing this change : — 



CaSO + + CH 4 = CaO + H 2 S + C0 2 + H 2 0. 



f>r :— 

 CaSO v + CH t = CaS + C0 2 + 2 H 2 = CaC0 3 + 

 H 2 S + H 2 0. 



As Mr. Sherburne Rogers remarks, these should be taken merely 

 as type equations, true for the higher hydrocarbons, since methane 



