DIFFERENTIATION PHENOMENA OF THE PROSPECT INTRUSION. 123 



connected with such folding the magma may probably be 

 moving slowly through a horizontal or vertical fissure for 

 many years. During the whole of this time a magma of 

 nearly uniform composition may be passing into the fissure, 

 but on the walls of the part first intruded the earlier pro- 

 ducts of crystallisation will have adhered and will continue 

 to do so, as long as the magma is passing. When it ceases 

 to flow, a magma which has already parted with some of its 

 early crystallising constituents consolidates between two 

 sheets of those constituents. 



There is another kind of differentiation which is probably 

 common but usually produces rocks on a small scale only. 

 A mechanical separation of the mother liquor may happen 

 after crystallisation has been nearly completed throughout 

 a mass of cooling magma by its being squeezed out by 

 earth movement along the planes of least resistance in the 

 surrounding rocks. Many granite-aplites, and other aplitic 

 rocks generally, such as bostonite, diorite-aplite. etc., 

 which penetrate the country-rock, may have been produced 

 in this way. A second mode of separation is that which, 

 I believe, produces the vast majority, if not all, of those 

 aplitic veins which are found traversing plutonic, and the 

 larger hypabyssal masses, whether basic, intermediate or 

 acid, though most frequently in the last. In any part of 

 the cooling mass where consolidation is more than half 

 completed, the earlier formed crystals must be adhering to 

 one another at so many points that they build up a kind of 

 solid net-work, or sponge. As the mass continues to cool 

 the crystals contract, and the spongy mass divides along 

 planes in three directions ; one parallel to, and the others 

 roughly perpendicular to, the cooling surface. At the same 

 time the mother liquor in the interstices of the crystal 

 network has no option but to percolate slowly out into 

 these cracks, under the pressure of the contraction, which 

 is squeezing it out like water from a sponge. There is 



