Smyth — Crystalline Rqoks of St. Lawrence County. 



495 



Tlie absence of any marked metamorphism of the limestone at its contact 

 with the pegmatite, is a feature observed at a number of localities in the case 

 of granites, and is of importance as showing that the mere absence of contact 

 products can not be taken as proving an absence of igneous intrusions. An 

 instance of importance is afforded in northern Rossi e, where an area of rather 

 massive gneiss is clearly intrusive in the limestone, w hile the latter shows no 

 change, unless, perhaps, a slightly coarser grain than usual. The gneiss, how- 

 ever, is, like the pegmatite of Gouverneur, rich in titanite near the contact. 

 In the same vicinity, some two miles north of Rossie village, a rather dark, 

 fine rock, in part massive and in part a gneiss, breaks through the limestone 

 with an irregular irruptive contact. At some points along the line of contact, 

 the metamorphism is confined to a zone not more than an inch wide, while at 

 other points great masses of pyroxene, scapolite, apatite, mica, etc., are found, 

 constituting an important mineral locality. The great variation in the amount 

 of metamorphism at different points does not seem to depend upon any 

 corresponding variation in the nature of the rocks involved. A probable 

 cause may be suggested as afforded by an uneven distribution of moisture, 

 the great masses of contact products being formed along channels affording 

 a favorable transmission for heated solutions, and, in some cases, doubtless 

 for gases as well. 



The two localities just described not only illustrate the variability in 

 the degrees of metamorphism but also afford further examples of gneiss, 

 igneous in origin and younger than the limestone. The development of 

 large masses of minerals in the last case naturally suggests the possibility 

 that other mineral localities of the region may show similar relations. As a 

 matter of fact, the association of minerals at many of these localities is such as 

 to lead to the inference that they have been formed by contact metamorphism. 

 In the case of the well-known localities east of Natural Bridge this was found 

 to be true, and at many other points similar relations are suggested, although 

 the structure is not so clear. 



Numerous pits from which minerals have been collected in quantity have 

 been visited, and in nearly every case they have been found to be opened on, 

 or close to, the contact between gneiss and limestone. Pyroxene, amphibole, 

 scapolite, feldspar and titanite are the common minerals, an association w hich, 

 taken with their mode of occurrence, is highly suggestive of contact meta* 

 morphism. In most cases examined the gneiss is not part of an extensive 

 area, but is rather limited and more like a sheet. For this reason it often has 

 the appearance of being interbedded, thus making the structural relations 



