198 R. Workman — Calcite as a Primary Constituent 



titaniferous pyroxene, titano-magnetite, perofskite, and apatite. The 

 basic rock with which the Alnb" titaniferous magnetite is associated 

 and into which it passes may therefore be described as a Jacupirangite, 

 sometimes with more apatite than the type-rock, at others with 

 addition of olivine, and always containing calcite. 



3. The Dtke Rocks. 



These are mainly Alnbites, nepheline-syenite porphyries, tinguaites, 

 nephelinites, and fine-grained limestones. The best known is the 

 melilite basalt described by Tbrnebohm as Alnoite. It is found in 

 narrow dykes cutting the sj^enites, limestones, and surrounding 

 gneisses, and it presents a striking difference from most melilite basalts 

 in carrying abundant large crystals of dark mica one or two inches in 

 diameter. Phenocrysts of biotite, titano-magnetite, aegirine-augite, 

 olivine, and apatite lie in a ground-mass of mica, idiomorphic melilite, 

 and calcite. Other primary constituents are perofskite, chromite, 

 garnet, and pyrrhotine. The conditions necessary for the formation 

 of abundant melilite in this rock in preference to nepheline seems to- 

 be a high percentage of lime and aluminium with a correspondingly 

 low percentage of silica and alkali. 



A fairly common leucocratic differentiation product is a fine-grained 

 tinguaite, which under the microscope shows a ground-mass of 

 beautiful idiomorphic nepheline and tiny felted needles of pale-green 

 aegirine-augite visible with the high power. 



Fine-grained limestone dykes of almost pure carbonate, with 

 a small proportion of silicates, are found in many parts, in the 

 nepheline-syenite as well as in the coarsely crystalline limestone. 

 They both cross other dykes, and are crossed by them. They have 

 all the appearance of true igneous intrusions. 



Conclusion. 



From the evidence which has been outlined above it is abundantly 

 clear that calcite functions as a primary igneous constituent of the 

 Alno complex. 



A few other instances of this phenomenon may now be mentioned 

 in passing. The Alno conditions are certainly repeated in a most 

 singular manner at Kuolajarvi, in Finland, for Ramsay & Nyholm 

 (10) have described a nepheline-syenite from this locality containing 

 aegirine, cancrinite, nepheline, orthoclase, sphene, and apatite, and, 

 associated with it, another rock containing oegirine, aegirine-augite, 

 biotite, nepheline, apatite, titano-magnetite, calcite, and zeolites. 

 The primary nature of the cancrinite has been established with great 

 care by Sundell (11) ; as at Alno this mineral disappears with 

 incoming of abundant calcite. 



Again, at Dungannon, in Ontario, Adams (12) has found calcite 

 in a perfectly fresh nepheline-syenite, enclosed in hornblende, 

 nepheline, plagioclase, felspar, and garnet, in such a manner that 

 he is led to regard it as primary. Another example has been 

 recorded by Sir Thomas Holland (13). In describing a group of alkali 

 eruptive rocks on Sivamalai, in the Charnockite Series, Madras, he 

 says : " The principal feature of interest is connected with the 



