C. H. Smyth, Jr. — Gdbbros in the Adirondack Region. 55 



are connected with the anorthosites previously known. Dur- 

 ing the past summer the writer examined a group of related 

 rocks, which, from their character, mode of occurrence and 

 location, seem to merit description. 



The locality is in the southwestern corner of Hamilton 

 County, between the hamlet of Morehouseville and Wilmurt 

 Lake, which lie about five miles apart on opposite sides of the 

 valley of the West Canada Creek. The hills bordering the 

 valley consist chiefly of two varieties of gneiss. One is a 

 rather acid rock composed largely of quartz and orthoclase, 

 with a peculiar spindle structure, giving to weathered surfaces 

 the appearance of partially decayed wood. The second variety 

 is of a brownish tint, which may be only superficial, and con- 

 tains hornblende in some quantity. Its foliation is of the 

 ordinary type and sometimes shows much crumpling. A 

 third variety of gneiss, usually nearly black, coarse grained, 

 often massive, and containing large lumps of garnet, is present 

 in more limited amount. The relations subsisting between 

 these gneisses have not as yet been determined. 



The rocks with which the present paper is chiefly concerned 

 are seen at several points between Wilmurt Lake and the 

 creek below, the first outcrop being about a mile and a half 

 down the road from the lake, and several others appearing at 

 various points within the next mile. Then there is a break 

 and no further outcrops appear till the village is reached. This 

 break is doubtless simply the result of the covering of expo- 

 sures by heavy stream deposits. The outcrops show a dark, 

 fine grained rock, forming irregular patches in the gneiss, with 

 the line of demarcation between the two very distinct. These 

 patches often have a tendency toward rectangular outline, and 

 may be nearly equilateral, or elongated into a dike-like form. 

 Their extent is usually limited to a few yards or rods. The 

 same rock occurs in what might be taken for an interbedded 

 layer in the gneiss, some fifteen feet thick and dipping 10° 

 south. At Morehouseville a coarser variety constitutes a knoll 

 three or four acres in extent. 



The true nature of the rock is best shown, so far as field 

 relations are concerned, in the small patches. It is seen to be 

 entirely diiferent from the surrounding gneiss in composition 

 and structure ; it cuts directly across the foliation of the gneiss 

 and the passage between them is as abrupt as possible ; occa- 

 sionally fragments of the gneiss are included in it ; and the 

 zone of contact between the two rocks is marked by a narrow 

 band differing in aspect from either rock and evidently the 

 result of contact metamorphism. From these facts it is clear 

 that the dark patches are intrusions in the gneiss, the bed-like 

 mass being an intrusive sheet. The large body of rock at 



