ART. 28 MINERALOGY OF TRIASSIC LIMESTONE SHANNON 15 



in the contact while at other places, where the same igneous rock 

 is in contact with identical limestone, no appreciable effect c'an 

 be found. Sometimes secondary minerals form at a considerable 

 distance from the intrusive as tabular bodies along fissures or as 

 pipes and in many places the garnetization follows single beds 

 for a long distance from the contact while the other beds of the 

 series are completely unaltered. The latter phenomena have been 

 explained by the proponents of the residual crystalHzation theory 

 as being due to impurities, capable of forming secondary silic'ates, 

 in the replaced beds while the unreplaced beds were devoid of the 

 constituents, notably silica, alumina, iron, etc., necessary to form the 

 garnet and other silicate minerals; but several authors have shown 

 that this does not hold true for in many cases it is the purer beds 

 which have been converted to silicates. 



The formation of these lime-silicate zones in limestone is in 

 almost all cases at the contacts of acid rocks, basic rocks very 

 seldom giving rise to such deposits. In the case of the Leesburg 

 quarry, however, the lime silicates are formed adjacent to a dia- 

 basic intrusion. Referring again to the preceding Goose Creek 

 paper, it m'ay be recalled that it was there concluded that the heated 

 magmatic solutions were released only after they had concentrated 

 in residual areas in the magma and had induced differentiation in 

 these areas so that the last rock to crystallize, preceding the release 

 of the solutions, was 'a quartz albite rock. The solutions, as such, 

 were thus in fact emanations from very acid rocks, despite the 

 small amount of the acid rocks and their derivation from a great 

 body of basaltic magma. These solutions were not stable in con- 

 tact with the already solidified bas'alt but reacted with it adjacent 

 to the fissures which formed channels for their escape, meta- 

 somatically replacing augite by diopside, plagioclase by albite and 

 sericite, and magnetite by titanite. It is these solutions which, 

 escaping along fissures in the limestone, accomplished the minei-'ali- 

 zation described in the present paper. This small-scale process of 

 elimination of concentrated solutions nz the final consolidation of 

 acid end products of differentiation is, if we may credit modern 

 petrologic theory, precisely what has happened in the larger bath- 

 olithic masses of relatively acid rocks. 



Emanations, by which is here meant principally water solutions, may 

 be given off in the earlier stages following the intrusion of a batholith 

 into its chamber, particularly if it be saturated with volatile ma- 

 terials, but it seems improbable that a high degree of saturation often 

 obtains. The water enters the magma chamber in solution in the 

 magma. There must be some essential difference in the behavior of 

 water in abyssal chambers crystallizing to give a plutonic rock of 

 granitoid texture and in a stock crystallizing at moderate depth. 



