of Igneous Rocks. 199 



presence of calcite in granular crystals, with apparently as much 

 right, as any of the others, to be considered as a primary constituent. 

 The crystals form isolated granules, and there is apparently no sign 

 of secondary decomposition, or structures which suggest its infiltration 

 into the rock." 



More numerous instances might be cited to illustrate the occurrence 

 of primary cancrinite in nepheline-syenites and allied rocks, but this 

 would lead too far afield. 



As a rule, in dealing with minerals which have crystallized to form 

 igneous rocks, one does not inquire further into their history after 

 ascertaining their primary nature. But in the case of calcite an 

 exception may be made : this mineral is, in the great majority of 

 cases, formed and concentrated under obviously non-magmatic con- 

 ditions ; and accordingly the question arises, has the Alno magma 

 incorporated a sedimentary limestone mass ? 



A very limited example of such incorporation has been demonstrated 

 by Dr. Teall (6, pp. 456, 460), at the contact of the Borolan syenitic 

 complex with the Durness dolomite. At one point a mixed rock. has 

 been found, consisting of orthoclase, calcite and segirine-augite, the 

 latter grown about crystals of diopside which has resulted from the 

 contact alteration of the impure dolomite. Absorption has also been 

 suggested by Daly (7) (8) to account for the presence of the calcite of 

 the Dungannon syenite, and this is not unreasonable, since the latter 

 occurs in close association with sedimentary limestones. 



From these cases, and from the well-marked stability of calcite in 

 pure limestones subjected to contact metamorphism, it is clear that 

 an alkali -syenite magma, in which the silica present is thoroughly 

 saturated with bases, may quite well absorb calcite without leading 

 to the dissociation of the mineral. But a sub-alkali magma may be 

 expected to react with any calcite which came to be incorporated, 

 liberating carbon dioxide, and at the same time yielding silicates rich 

 in calcium ; such reactions would, of course, only proceed until the 

 magma had been desilicated to a sufficient extent to render any 

 further calcite absorbed immune from attack. It is quite conceivable, 

 for instance, that through the withdrawal of silica in combination 

 with lime, a normal granitic magma might eventually be converted 

 into a variety of felspathoid syenite. From considerations of this 

 character, combined with Lcewinson-Lessing's views (9) that sometimes 

 the assimilation of a small quantity of foreign matter in a magma is 

 sufficient to induce differentiation to a greater or less extent, Daly has 

 been led to regard alkaline rocks in general as derived from sub-alkali 

 magmas through the absorption of limestone, followed by induced 

 differentiation. This view he supports by calling attention to the 

 small relative bulk of alkali magmas, and to the suggestive association 

 of the latter in several cases with limestones, under circumstances 

 which admit of the possibility of important absorption. For Daly, 

 Alno and Finland offer clear evidence, not merely of the absorption 

 of limestone, but also of the consequent evolution of alkali magmas. 



In applying this line of argument to Alno, however, the question 

 arises, where did such a limestone as is postulated ou the absorption 



