56 C. H. Smyth, Jr. — Gabbros in the Adirondack Region. 



Morehouseville gives only limited indications of its relations to 

 the gneisses and the determination of its intrusive character is 

 based upon its composition and structure and resemblance to 

 the rocks of the smaller patches, rather than upon its mode of 

 occurrence. This being the case, the character of the rock 

 from the undoubted intrusions will be first considered. 



Specimens from these outcrops show variations in color from 

 very dark blue-gray, to a lighter gray with brownish tinge. 

 The grain is fine and even, with little variation from point to 

 point. There are, however, decided differences in the struc- 

 ture of the rock at different outcrops, as in some it is almost 

 completely massive, while in others it is distinctly gneissoid. 

 In every case the foliation is parallel to the foliation of the 

 enclosing gneiss, and both in the field and under the micro- 

 scope shows plainly that it is a result of pressure and not a 

 flow structure. Weathering produces a light brown crust 

 which has not been found of any great thickness, but which 

 greatly obscures the superficial difference between the intru- 

 sives and the surrounding gneisses. Differential weathering 

 has not proceeded sufficiently far since glaciation to cause any 

 marked difference in elevation betw T een the two rocks, but 

 there is often a slight projection of the intrusive patches 

 above the gneiss. Jointing is more perfectly developed in the 

 former than in the latter, giving an appearance like that of 

 many dikes of diabase and related rocks. 



The darker variety of the rock very strongly resembles 

 some of the finer grained specimens of the Baltimore hypers- 

 thene gabbro described by Professor G. H. Williams.* In fact 

 this resemblance is so strong that it is sometimes difficult to 

 distinguish from each other specimens from the two localities. 

 The same resemblance exists somewhat in the composition and 

 structure of the rocks as seen under the microscope, but in a 

 less marked degree. Sections of the Wilmurt rock show a 

 holocrystalline granular aggregate of plagioclase, hypersthene, 

 monoclynic pyroxene, hornblende, a little biotite, and magne- 

 tite. None of these minerals have crystal outlines, being in 

 irregular grains which range, as a rule, from -1 to # 5 mm in diam- 

 eter. A rather conspicuous feature is the absence of apatite, 

 which is seen only rarely and in very small quantity. In a 

 single specimen several grains of garnet are shown. Varia- 

 tions in the rock result from the different proportions of con- 

 stituents present, and, in less degree, from differences of 

 structure. In nearly all specimens the minerals are extremely 

 fresh, showing almost no effects of weathering. 



* The Gabbros and Associated Hornblende Rocks occurring in the neighbor- 

 hood of Baltimore, Md. U. S. G. S., Bull. 28. 



