270 SMYTH — ROCKS OF NORTHWESTERN ADIRONDACK REGION. 



labradorite, but in portions of the rock having an abnormally . small 

 amount of ferro-magnesian minerals and containing some quartz there is 

 a much more acid plagioclase and probably some orthoclase. The feld- 

 spar of all specimens is entirely free from the fine inclusions so common 

 in the feldspar of most gabbros. 



The feldspar is often much altered to kaolin and to muscovite, 

 more rarely, to scapolite. This latter change is interesting, recalling the 

 group of scapolite-diorites and " gefleckter gabbros." It is also sug- 

 gestive in connection with the scapolite rock of Gouverneur previously 

 described by the writer.^ A peculiar feature of the gabbro, considering 

 its rather basic character, is the almost complete absence of magnetite 

 and ilmenite, these minerals being rarely seen in microsections. 



Effect of Contact. — ^The contact phenomena are important only where 

 the gabbro cuts the limestone, as it has little or no effect upon the gneiss. 

 The limestone is converted into a coccolitic mass, chiefly green pyroxene, 

 but containing considerable quantities of garnet, scapolite and sphene, 

 the latter often in perfect, though very small, crystals, the other minerals 

 being in grains. This change is shown both on the border of the mass 

 and in inclusions, some of which are changed throughout to the aggre- 

 gate described. 



Relation of the Gabbro to Ghranite Intrusions. — As the gabbro occurs in 

 the line of granite intrusions above described, the relation between the 

 rocks is a matter of interest. If the acid granites of Gouverneur pass 

 into quartz-free diorites it might be thought that the gabbro is a basic 

 phase of these more basic granites. That this is not the case is con- 

 clusively proved by a contact between the gabbro and the granite. The 

 contact is sharp, with no transition, and clearly irruptive. The structure 

 of the contact does not, however, prove the relative age of the intrusions, 

 but the extreme fineness of the gabbro and its exceptional aspect under 

 the microscope indicate that it is the later of the two rocks. 



The Gahbro as a Basis of Compariso7i. — This gabbro is of particular 

 interest in affording a basis of comparison for rocks from other portions 

 of the region, where the field relations are less clear. The black gneiss 

 from southern Hamilton county, recently described by the writer, f 

 affords an instance. This rock was referred to as probably belonging to 

 the gabbro series, although no positive proof of this was at hand. While 

 not, of course, affording such proof, the Pitcairn gabbro gives great sup- 

 port to the supposition named, as there is a strikingly close resemblance 

 between many portions of the rocks, though the Hamilton county rock 



* Trans. New York Acad. Sci., vol. xii, p. 215. 



fOn Gabbros in the .southwestern Adirondack region. Am Jour. Sci., vol. xlviii, p. 54. 



