MOVEMENTS IN CRYSTALLIZING MAGMA 261 



gested by Barrow. First, horizontal compression of a laccolith, 

 crystallized to about 80 per cent, might yield an upper differentiate 

 of liquid;' the effect on the crystallizing residue is clearly described 

 as follows: "The horizontal dimensions are shortened in conse- 

 quence of closer packing of crystals." Second, horizontal compres- 

 sion of a sheet in which the top and bottom had partly crystaUized 

 might result in a squeezing of the hquid into the central zone ; here 

 again the crystalline residue will have its horizontal dimensions 

 shortened. Third, there may be a warping of the walls of a sheet; 

 the forces which are expected to compress part of the sheet to such 

 a degree that other parts are thickened are not described. It would 

 seem that there is no reason to assume a central accumulation of 

 the liquid in this case, any more than in the case of a laccolith, 

 where an upper layer is supposed to accumulate. Both sheets and 

 laccoliths have competent roofs. Both are likely to have their 

 upper layers solidified early. 



There are thus outhned three cases in which Bowen finds no 

 mechanical difficulty in imagining an action like a filter press. Two 

 fundamental objections may be raised and should be answered 

 before final acceptance of the idea. The first has to do with the 

 structures left in the crystalline mesh after expulsion of the fluid; 

 the second is a matter of the completeness of the separation, and 

 the volumes of the separated parts. 



Structures. — These three types of filter-press action occur 

 when the mass is 80 per cent or more crystalline. Such a mass 

 would have the crystals pretty well locked together. Uniform 

 spheres closely packed take up only about 75 per cent of the space 

 in which they are packed. Packed angular grains may occupy as 

 little as 50 per cent. Even 5 per cent further growth would result 

 in a fairly firm bond between adjacent crystals. If now the mesh 



' Bowen carries the idea too far when he attempts to apply it to the Duluth lopo- 

 lith. Lateral thrust on a sunken basin would hardly tend to dome up the roof. Fur- 

 thermore he makes an inaccurate copy of a map published with a plain statement that 

 the area on which he bases his argument had not been mapped in detail. 



Editorial Note: In this connection it may be said that the map which Dr. 

 Bowen states that he copied is the map printed as Fig. 6 in the Journal of Geology, 

 Vol. XXVI, p. 446. The map which Dr. Grout apparently has in mind in this 

 connection is the map reproduced as Fig. i, Vol. XXVI, of the same Journal, p. 628. 

 The differences between the two are very slight. 



