60 C. H. Smyth, Jr. — Gahlrros in the Adirondack Region. 



mine whether the same regularity of orientation persists in 

 the finer tongues, but such seems often to be the case. Again, 

 the tongues wander irregularly through the feldspar, often 

 extending between adjacent individuals. Figure 1 shows the 

 first step toward this structure, figure 2 its more complete 

 development though not approaching the fineness and com- 

 plexity often seen. The extreme cases are often accompanied 

 by an alteration of the feldspar to muscovite. 



This stringing out of the hypersthene shows a most inti- 

 mate connection with the development of gneissoid structure. 

 When there are only slight traces of the latter structure the 

 hypersthene occurs merely as irregular grains, but as soon as 

 parallelism of constituents becomes marked the stringing out 

 begins and becomes more and more conspicuous as the gneis- 

 soid structure increases. It seems a necessary inference that 

 the phenomenon is a result of metamorphism but the rationale 

 of the process is not clear, and it seems best to defer any 

 attempt at explanation until more data are available. 



The contact between the gabbro and the gneiss is quite dis- 

 tinct in hand specimens, being marked by a greenish gray 

 band about 2 mm wide. Under the microscope the contact 

 shows a narrow zone made up almost wholly of the pyroxenes 

 with some biotite. Just within this band the gabbro consists 

 chiefly of feldspar, and then gradually assumes its ordinary 

 composition, though often containing an unusual amount of 

 hornblende. The character of the contact zone is clearly clue 

 to the fact that the pyroxenes crystallized earlier than the 

 feldspar and attached themselves to the solid face of the gneiss, 

 leaving the adjacent layer of gabbro composed of nearly pure 

 feldspar. The pyroxenes of the contact zone are almost com- 

 pletely altered to a green chloritic product, to which is due the 

 color of this zone as seen in hand specimens. The gneiss 

 shows no perceptible mineralogical changes at the contact, 

 though its color is altered to a bluish gray tint. 



The chemical composition of the gabbro is indicated by 

 analysis I. The material analyzed was taken from a very fresh, 

 massive specimen which showed under the microscope about 

 the average proportions of minerals. 



Analysis II gives the composition of a fresh specimen of 

 the Baltimore gabbro described by Professor "Williams* and 

 III gives the results obtained by analyzing a mixture, in equal 

 amounts, of specimens of this rock from twenty-three different 

 localities.f The similarity in the results of the three analyses 

 is striking and confirms the conclusion that the gabbro at 

 Wilmurt is closely related to the Baltimore Rock. 



* Op. cit, p. 37. f OP- cit., p. 39. 



