200 R. Workman — Calcite as a Primary Constituent 



theory come from ? Hogboni is uncertain as to the age of the 

 complex, which he considers probably pre-Cambrian, but possibly 

 Devonian, so that it is conceivable that limestone may have been 

 derived either from the Archaean, or, as a result of 'stoping', from 

 overlying ' orthoceras kalk ', which latter has now been entirely swept 

 away by erosion from the surrounding country. But no limestones 

 are known in the Archaean of this region, and the ■ orthoceras kalk ' is 

 very impure, whereas the Alno limestone, apart from normal igneous 

 minerals (as opposed to contact minerals), is exceptionally free from 

 contamination. 



Hogboni' s conclusion is, that the calcite has crystallized from the 

 magma in a manner exactly analogous to the other minerals. He 

 suggests that the syenites and limestones may have been separated 

 during the general differentiation of which there is so much evidence 

 among the dyke-rocks of Alno. But the two cases are not exactly 

 analogous, for the various dyke-rocks present differ from one another 

 in regard to the nature of the minerals, while the syenites and 

 limestones are distinguished by their quantitative hut not by their 

 qualitative mineralogical constitution. It is important to notice 

 that the minerals in these plutonic rocks have not concentrated in 

 a definite order; as far as I have been able to judge no definite law 

 for the order of crystallization can be given here. Therefore there is 

 no differentiation due to a regular order of crystallization. 



But we shall not digress further into the interesting questions 

 raised by the presence of primary calcite in the igneous complex of 

 Alno. The main purpose of this paper has been fully attained if it 

 has enabled the reader to form some conception of the unwonted 

 experiences and pleasant surprises which awaited us in this out of 

 the way island, whither we were led by Professor Hogbom. 



References to Literature. 



(1) HOGBOM, A. G., " Ueber cles Nephelinsyenitgebiet auf der Insel Alno " : 



Sveriges Geologiska TJudersokn, ser. C, 1895. " The Igneous Eocks 

 of Bagunda, Alno, Eodo, and Nordingra " : Geol. Foren. Forhandl., 

 1909. 



(2) HORNE, J., & Teall, .T.J. H., "On Borolanite " : Trans. Eoy. Soc. 



Edinb., vol. xxxvii, pt. i, 1892. 



(3) Shand, S. J., "On Borolanite and its Associates in Assynt " : Trans. 



Edinb. Geol. Soc, vol. ix, pt. v, 1910. 



(4) Geikie, A., & Teall, J. J. H., " On the Banded Structure of some 



Tertiary Gabbros in the Isle of Skye " : Q.J.G.S., 1894. 



(5) Derby, 0. A., " On the Magnetite Ore District of Jacupiranga, Brazil " : 



Amer. Jour. Sci., 1891. 



(6) Teall, J. J. H., in "The Geological Structure of the North-West Highlands 



of Scotland " : Mem. Geol. Surv., 1907. 



(7) Daly, E. A., " The Mechanics of Igneous Intrusion " : Amer. Jour. Sci., 



vol. xxvi, 1908. 



(8) " Origin of the Alkaline Eocks " : Bull. Geol. Soc. Amer., vol. xxi, 



1910. 



(9) Lcewinson-Lessing, F., Compte Eendu Seventh Internat. Geol. Congress, 



St. Petersburg, 1897. 



(10) Eamsay, W., & NYHOLM, F., " Cancrinite Syenite, Kuolajarvi " : Bull. 



Comm. Geol. Finlande, 1895. 



(11) SUNDELL, J. G., ibid., 1905. 



