696 , CLARENCE N. FENNER 



imparted by the presence of such volatile matter — -increase of 

 mobility and greater chemical activity — -will be retained. 



The ease of flow manifested by the New Jersey granite which 

 has been described is therefore quite understandable; nevertheless 

 it seems to have retained a quite effectual degree of viscosity, as 

 has been pointed out in describing the field phenomena. The 

 facility, then, with which magmatic material seems to have been 

 transfused into the adjacent strata is not at once intelligible. The 

 difficulties in harmonizing the phenomena which are exhibited will 

 be better appreciated by a calculation of the rate at which a fluid 

 under pressure enters minute pores. For instance, an approximate 

 calculation by Poiseuille's formula shows that if we assume a liquid 

 having a coefficient of viscosity of i . o (about one-tenth that of 

 glycerine at 19° C. or 100 times greater than that of water at the 

 same temperature) and conceive it to be pressed with a force of 

 810,000 gm. per square cm. (corresponding to an overhead load of 

 about 3 km. of rock strata) against a surface of i sq. cm. pierced 

 with 1,000 holes, each having a radius of o.oooi cm., and suppose 

 the thickness of the partition to be i dm., then in one year only 

 o. I c.c. would ooze out on the far side. 



The assumptions made are far from exact. The form and 

 disposition of the pores in rocks do not correspond to the straight 

 cylindrical tubes to which Poiseuille's formula closely applies, and 

 the exact degree of viscosity is unknown, but the most important 

 factor is the diameter of the pores, for the flow is about as the 

 fourth power of this dimension. Thus if we had assumed pores 

 of a radius of 0.0004 cm. (probably an excessive estimate of size 

 for the pores of ordinary rocks under heavy load) we should have 

 derived a value of 25 c.c. per year. 



It seems, on the whole, that we can expect little movement 

 of the magma as a unit into minute pores, and we shall have to seek 

 an explanation of its transfusion into the adjacent rock from other 

 considerations. 



Among the phenomena shown in the field several facts merit 

 emphasis. As I have already mentioned, the introduction of the 

 magma seems to have been effected with remarkably little dis- 

 turbance of the previously existent relations of the original layers. 



