DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



members of the Camples, coarse hornblende-peridotite and homblende-gabbro, 

 eut by medium-grained gabbro. All of these rocks are more or less gneissic 

 but the dike itself is neither schistose nor otherwise visibly affected by dynamic 



The dike is highly conspicuous through the whole length of its outcrop. 



The salient feature is the studding of its surface with hundreds of nodules 



which form about 40 per cent of the rock. (Plate 37.) These nodules are 



ellipsoidal or potato-shaped, measuring from 3 cm. to 6 cm. in diameter. They 



■ resist destruction by weathering more effectively than their matrix, so that they 



•stand out prominently on the ledges. 



The nodules are light-green, granular aggregates of interlocked olivine 

 ■• crystals from 1 mm. to 10 mm. in diameter. Harely, a small anhedron of pyrox- 

 ene, probably diallage, appears as an accessory, interstitial constituent of the 

 nodule. No other primary mineral except abundant minute microlites of 

 ehromite or picotite, is present. The olivine is often surprisingly fresh but 

 along cleavage cracks it has gone to serpentine, talc, magnetite, and tremolitic 

 amphibole. In other cases these secondary minerals compose most of the nodule. 

 The deep brown inclusions of spinel or chromite have the usual sharp crystal- 

 form and parallel arrangement in the individual grains of olivine. 



The matrix is much darker-coloured than the nodules and is considerably 

 more altered. It was originally composed chiefly of a granular aggregate of 

 hypersthene, with which a green hornblende was associated. Now, however, 

 the matrix is mostly a felted mass of colourless amphibole, often assuming a 

 greenish tint like that of actinolite. Much magnetitej bastite, talc, and some 

 serpentine also occur in the felt. Limonite often stains the thin section. The 

 hypersthene has the usual colour, pleochroism, and other properties and has a 

 great abundance of interpositions which, under the microscope, have the same 

 optical properties as the spinel-like microlites of the olivine. Minute granules 

 of what appears to be magnetite also occur in normal parallel arrangement in 

 the residual cores of the hypersthene. The deep green hornblende seems without 

 question to be of primary origin and thus of origin and nature quite different 

 from those of the tremolitic amphibole. 



Between the nodules and their matrix there is almost invariably a kely- 

 phitic shell of a colour yet lighter than that of the nodule. The shells vary in 

 thickness from a couple of millimetres or less to 15 mm. They are composed 

 chiefly of tremolite and magnetite, the amphibole prisms often radiating out- 

 ward from the nodule. Some talc and serpentine also appear in these ' reaction 

 -.Tims.' 



Plate 37 shows the relations of nodule, reaction-shell, and matrix. The 

 kelyphitic phenomenon is well known to petrographers and needs no further 

 description. The peculiarity of the dike consists in the fact that it is a peri- 

 dotite crowded with large olivine nodules. So far as known to the writer no 

 similar dike has been described in petrographic literature. It may be noted 

 that the nodules preserve their average size throughout the cross-section of the 

 dike; the average is not affected by the proximity of the walls. In that respect 

 -also the conditions of crystallization were unusual. 



