22 PROF. W. C. BROGGER ON THE [Feb. 1 894, 



can be proved to be incorrect. I had myself believed till now 

 that contact-inetarnorphism in strata of the same nature, by other- 

 wise identical conditions, has generally given rise to the same 

 alteration-products — without regard to the ' chemical quality ' of tbe 

 eruptive rock in question. The development of the contact-meta- 

 morphism in Gran seems now to prove that this opinion was 

 erroneous. 



The short time at my disposal does not allow of a detailed 

 description of all the observations upon which I found my altered 

 opinion. I shall only mention a single fact. 



Along the boundary of the basic plutonic rocks of Gran, all of 

 which are comparatively rich in magnesia and iron oxides, the more 

 highly altered hornstones of the O^/c/m-shales show an essential 

 percentage of hypersthene. I had previously mistaken the mineral 

 for andalusite ; it occurs as innumerable small prismatic crystals, 

 but is easily distinguished by its optical characters. To make 

 assurance doubly sure, I have, with considerable difficulty, isolated 

 a small portion of the hypersthene, and have had it analysed by 

 Herr L. Schmelck. The analysis gave : — 



Si0 2 48-10 



FeO 2228 



MgO 2183 



CaO 220 



94-41 



The loss is due to a mishap in the course of determination of the 

 silicic acid ; adding 5*56 per cent. Si0 o , the above analysis agrees 

 exactly with the composition of an hypersthene, in which the 

 molecular proportions are 



PeO : (MgO, CaO) = 1:2. 



Now the very same strata of the Ogygia-sh&les, 7 kilometres 

 (4 5 miles) east of Solvsberget, are altered into hornstones by the 

 influence of the immense masses of quartz-syenite (nordmarkite) ex- 

 tending over the whole district between Gran and Christiania. I have 

 examined a number of specimens of these hornstones from Ranasen 

 and other localities. In none of them have I discovered the slightest 

 vestige of hypersthene. As is well known, hypersthene or ortho- 

 rhombic pyroxene is, upon the whole, never observed to have been 

 produced by contact-metamorphism alongside abyssal rocks. 



It seems, therefore, that in this case the basic magma of the 

 olivine-gabbro-diabase, as compared with the acid magma of the 

 quartz-syenite, must have influenced, in a peculiar manner, the 

 argillaceous shales of etage 4 a a by altering them into hypersthene- 

 bearing hornstones. Whether this special influence is due to a 

 transfer of magnesia and ferrous oxide from the magma, or not, is a 

 problem which can only be settled by a series of analyses as yet 

 unfinished. 



In other respects, too, the contact-metamorphism alongside the 

 above-described basic rocks is of interest ; but, as this question is 



