M. Keilhau's Theory of Granite and other Rocks. S97 

 masses to bring forward of those which occur in the limestone 

 and clayslate, and also in the sandstone of the transition ter- 

 ritory of Christiania. These masses present porphyries which 

 on the one side approach the type of this rock, which is dark 

 coloured, and contains no quartz, and which forms the prin- 

 cipal mountain-rock of the great porphyry districts ; and on 

 the other side approach the red eurite-porphyry, which occurs 

 only in a subordinate manner in the stratified districts. Among 

 the varieties of the first kind, Von Buch found the prototype of 

 his rhombic porjjhyry ; and a variety of Brongniart's brownish- 

 red porphyry {porphyre brun-rouge) or melaphyre, which that 

 author says is to be found on Tyveholm, close to Christiania 

 (Brongmarfs Tableau, p. 350, and Classification, p. 107), be- 

 longs also to this division. The masses, consisting of rhombic 

 porphyry, exhibit in Christiania Dal, a third sort of form pecu- 

 liar to themselves, when we regard eurite-porphyry, with its 

 mode of occurrence in beds, as the first kind, and greenstone, 

 with its mode of occurrence in veins, as the second. In the 

 vicinity of the town we see the rhombic porphyry partly in 

 pei'fectly irregularly bounded masses, lying in a series of strata 

 composed of clayslate and limestone. These masses are for the 

 most part extended in the same direction as the greenstone 

 veins ; but they are comparatively short, and never exhibit 

 even and reciprocally parallel salbandes* like the greenstones ; 

 on the contrary, we observe the rhombic porphyry branching 

 out into the bounding rock in thick wedges, or broad round 

 offsets. In other localities this porphyry presents itself in pretty 

 regular veins, but it is there that, in its petrographical charac- 

 ters, it approaches the eurite-porphyry. An example of rhom- 

 bic porphyry as a vein in the primary rocJcs themselves, where 

 the latter are no longer covered by the transition series, is to 

 be seen at two places ; and at least in one of these it is 

 manifest that the vein extends without interruption for a 

 long distance into the transition territory, where it appears 

 in strata of limestone and clayslate. The same vein, — and one 

 of the places where this is to be seen is on the promontory of 

 Noesodden, — exhibits another extremely remarkable relation, 



* The two surfaces which form the walls of a vein are termed saalbandes of 

 lalbandts. — £di t. 



VOL. XXIV, NO. XLYIII. — APBIL 1838. D d 



