TYPES OF LACCOLITHS 411 



whole before differentiation into zones, with lower Si0 2 , and with oxides 

 of Fe, Mg, and Ca either equal to or preponderating over the alkalies. 

 The third is syenitic or foyaitic (also applied to the general magma) 

 rather low in Si0 2 , but with either K 2 or Na 2 or both preponderating 

 over the bivalent oxides. (This applies especially to Na 2 0, since few or 

 no examples of essentially potash-rich laccolithic , masses. are known or 

 have been studied.) These last two are uniformly granitic in structure.* 

 Of course, it is not asserted that this is the invariable rule, nor that these 

 three are the only possible types, but the few cases which have been in- 

 vestigated in sufficient detail seem to bear it out in general, though there 

 are apparent exceptions. 



Assuming that such a distinction between the three classes exists in 

 fact, or is at least generally applicable, it is suggested here that the 

 magmas of the first (Henry Mountain) type either approach in composi- 

 tion a eutectic mixture, analogous to that suggested by Teall,t or else that 

 the rocks represent closely the chemical character of the — or a — magmatic 

 solvent, the solution being so dilute that the solvent crystallized in an 

 approximately pure condition, the small amount of solute either being 

 mechanically caught and crystallized with it, or forced inward and 

 solidifying as a very small basic core, yet to be discovered, at the center 

 of the mass. In the other two types the solvent was either (Square 

 Butte type) rich in iron oxides, magnesia and lime, and the solute alka- 

 line and aluminous, or (Magnet Cove type) the converse, the solvent 

 being composed essentially of silica, alumina, and alkalies, and the solute 

 consisting mainly of bivalent oxides. In a possible fourth t} 7 pe, of gabbro 

 or peridotitic magma, the mass would again be quite uniform, since here 

 (analogously with the Henry Mountain type) the basic solvent is largely 

 in excess. 



These four types, it must be mentioned, are only given as examples, 

 and are not to be considered as exhausting the possible types of differ- 

 entiation. These may be very various, and, of course, dependent on the 

 composition of the magma and the physical conditions of cooling. These 

 four are, however, probably the most common and important, though a 

 possible fifth, that of a gabbro magma, with anorthosite and pyroxenite 

 zones, may be added. 



This view, which, it must be confessed, is only a working hypothesis, 

 explains otherwise apparently discordant facts in many cases, though it 

 by no means excludes the possibility of other processes being involved. 

 It is generally believed that the laws governing dilute liquid solutions 

 are quite distinct from those involved in more concentrated ones. It is 



* On this point, as regards volcanic rocks, see Jour. Geol., 1895, vol. iii, p. 62. 

 f Teall : Brit. Petrog., p. 402. 



