446 BOAVEX AXD AUEOUSSEAU FUSION OF SKDIMEXTARY ROCKS 



the geothermic gradient given by Hobbs, we find tliis temperature to l)e 

 about 315 degrees centigrade, which is scarcely acceptable as a temper- 

 ature at which fusion of shale, even retaining its water, is to be credited. 

 Particularly is this true when the nature of the process postulated is 

 considered. The shale is supposed to fuse as a result of relief of pressure. 

 Xow, it is true that most silicates and silicate mixtures expand on melt- 

 ing, and that for any such mixture there is probably a combination of 

 temj)erature and pressure conditions such that an isothermaP relief of 

 pressure would cause melting, though whether such a combination of 

 temperature and pressure conditions obtains anywhere in the earth's 

 crust is a question. But, even assuming that such a combination does 

 obtain for anhydrous material, as soon as a volatile component enters 

 into the system the whole aspect of affairs is changed. Thereafter the 

 principal effect of lowering of pressure will be to decrease the amount 

 of volatile component which the rock matter is capable of containing, 

 with a consequent raising of the melting temperature. Plainly the great 

 lowering of melting temperature through the presence (under pressure) 

 of volatile matter, and the lowering of melting temperature by lowering 

 of pressure in anhydrous silicates, can not be considered as working 

 together in any hypothesis of magma genesis. 



The only promising method of fusing shale would appear to be to have 

 it exposed, retaining its water, to a temperature of some 850 degrees 

 centigrade or more. If this temperature is to be the result of the normal 

 thermal gradient, the requisite depth would presumably be about 17 

 miles. Under these conditions most shale would probably be fused, but, 

 be it noted, relief of pressure would tend to cause it to crystallize. 

 Whether shales ever become buried to such depths is questionable. 



Under the influence of heat brought from the depths by basic magmas 

 shales have sometimes been raised to some such temperature as that just 

 mentioned and have been fused. An analysis of such fused shale (phyl- 

 lite) is given in Table IV, number 5. As will be noted, the conditions 

 were such as to permit retention of water, which is probably essential to 

 the fusion of shale by any magma. The process is probably one of no 

 great quantitative importance, though possibly shale may, on occasion, 

 be thus refused in sufficient amount to give rise by reintrusion to distinct 

 bodies of small size. To such action may, perhaps, be attributed the 

 abnormal volcanic necks in Lake Janisjarvi, Finland, described ])y 

 Eskola.=^ 



•^ The word isothermal should be carefully noted. It is doubtful whether any such 

 action could be realized in the earth's crust. 

 » Bull. Com. Geol. Finlande, no. 55. 1921 



