246 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



serpentine results from the alteration of a mineral older than the pyroxene. 

 Most probably this mineral was olivine, though no positive statement to 

 that effect can be made. This pyroxene rock also contains an exceedingly 

 large quantity of apatite. No analysis was obtained of this facies, but the 

 microscopical characters enable us to place it as a gabbro (possibly olivine- 

 bearing) and to consider it, like the bronzite-norite, as a facies of the pre- 

 dominant hornblende-gabbro. 



This same exposure of gabbro is cut by a coarse peridotite (wehrlite), 

 a description of which will be found on p. 253. In this peridotite there is 

 found a narrow strip of rock, about 2 inches wide, which is presumed to be 

 either an inclusion or a very narrow dike in the peridotite. The exposure 

 does not admit of its relations being determined more accurately. The 

 presumption is that it is a dike. Whether an inclusion or a dike, it is 

 younger than tlie massive hornblende-gabbro forming the main exposure. 

 In this respect it corresponds to the gabbro ju.st described (p. 243) as cutting 

 the normal gabbro. 



This dike rock is macroscopically a fine-grained, granular, dark-gray 

 rock. The microscope shows it to be veiy fresh, porphyritic in texture, 

 and composed of phenocrysts of bronzite lying in a finely granular ground- 

 mass of plagioclase, hornblende, and orthorhombic and monoclinic pyroxene. 

 No quartz whatever was found associated with these minerals. . The pla- 

 gioclase is the usual kind, labradorite. Some unstriated feldspar, possibly 

 orthoclase, was also observed, though in very small quantity. The bronzite 

 is in narrow prisms which reach a length of 1.23 mm. Commonly they 

 have well-developed transverse partings. It is worthy of note that a few 

 of the crystals contain the brownish platy inclusions so common in hyper- 

 sthene. There is, however, no relation between the color and pleochroism 

 of the mineral substance and these brownish plates. The bronzite is very 

 clear, with weak color, showing a scarcely distinguishable greenish tinge 

 for the rays vibrating parallel to C, Avith yellowish for the rays parallel to a 

 and t. In one case the bronzite was seen altering to a greenish-yellow 

 fibrous aggregate of serpentine. In the groundniass this same orthorhombic 

 pyroxene is represented by irregular grains. The hornblende is the usual 

 reddish-brown kind, but differs froni that seen in the other gabbros of this 

 district in that it contains ilmenite inclusions only, without any rutile or 

 anatase. It is in anhedra. A faint-greenish pyroxene occurs in irregular 



