272 SMYTH — ROCKS OP NORTHWESTERN ADIRONDACK REGION. 



Diana-Pitcairn limestone belt. The line of junction of these two rocks 

 is clearly defined, but such is not the case on the other sides of the gabbro, 

 as the rock undergoes decided modification, which renders its delimita- 

 tion a matter of difficulty. The clearly recognizable mass of the rock has 

 been traced from near Natural Bridge to a point three or four miles east 

 of Harrisville, a distance of nearly or quite twenty miles. On account 

 of the modifications mentioned the southern boundary of the mass is 

 uncertain, but an average width of about four miles may be taken as a 

 minimum. 



Appearance in the Field. — Over a large proportion of the gabbro belt 

 outcrops are abundant. The rock surface usually presents flowing con- 

 tours, with something the aspect of roches moutonnees. This is particu- 

 larly trae in the eastern part of the belt, where the gabbro rises into con- 

 siderable hills. In the vicinity of Natural Bridge the surface is flatter. 



In many cases it is not easy to distinguish a weathered portion of the 

 gabbro from coarse gneiss, but, as a rule, it is more massive and shows 

 the prevalence of large feldspars. The most favorable exposures for an 

 examination of the surface are those which have been smoothed by glacial 

 action and since preserved from decay. Such a surface is usually a very 

 light drab or gray, and has a somewhat porphyritic aspect ; that is to 

 say, the rock appears to consist largely of distinct feldspar individuals, 

 ranging from a fraction of an inch up to an inch or more in diameter, 

 held together by a varying amount of finer material. 



In some cases the appearance of the surface recalls a rather fine con- 

 glomerate of great uniformity of grain and composition. Usually on 

 such surfaces no constituent other than the feldspar can be determined, 

 although dark minerals are shown in small grains. 



Characteristics of the Feldspar. — In form the feldspar grains may be 

 rounded or nearly rectangular, approaching their crystallographic out- 

 lines. A zonal structure is often shown, the marginal portion being nearly 

 white, while the interior is gray or brown. The white band is about 

 one-fourth as wide as the individual and the passage into the dark core 

 is rather abrupt. There is a marked difl'erence in the resistance ofl'ered 

 to decay by these two portions of the feldspar. The dark area decom- 

 poses much the more readily, so that on glaciated surfaces which have 

 been slightly weathered the feldspar grains have a depression in the 

 central portion, surrounded by a rim of the white material. This mode 

 of weathering is a great help in determining the true nature of doubtful 

 exposures. Where the rock has been recently blasted it has a decided 

 gray color, sometimes with a greenish tinge. Occasionally it becomes 

 pink or red without deviating otherwise from the normal character. As 

 is natural, such a surface has a more granular aspect than those which 



