30 NEW YORK STATE MUSEUM 



some rather puzzling features. Its relation to the surrounding rock 

 is not so well known, but the best evidence points to its classification 

 with the other gabbros. Admittedly, however, some of the rock 

 of this area may belong with the so-called basic syenite. For most 

 part it is a very basic, gabbroic-looking rock, sometimes rather 

 massive and coarse grained with feldspar and hornblende or hypers- 

 thene individuals up to an inch long, and at other times not so 

 coarse but streaked or almost banded, due to layers Of amphibolite. 

 All phases of the rock are much granulated and distinctly gneissoid, 

 the coarser grained portions being least so. In spite of these 

 strongly developed secondary features, a coarse diabasic texture is 

 still frequently noticeable. Xumiber 48 of the table shows the 

 composition of the typical medium-grained rock. Labradorite is 

 the most abundant feldspar, while garnet is common and probably 

 secondary. The most probable explanation of these features seems 

 to be that a large mass of coarse-grained gabbro was here in- 

 truded and that, under excessive subsequent pressure, varying 

 degrees of foliation and granulation have been produced.^ 



Diabase 



Six diabase dikes have been located within the quadrangle, and 

 these are very similar to others so well known throughout the 

 Adirondack region. They are of late Precambric age and wholly 

 nonmetamorphosed. All but one of the dikes strike nearly north- 

 east-southwest, which is quite the rule for northern New York. 

 With one exception, these otherwise fine-grained rocks are more 

 or less porphyritic, due to the presence of phenocrysts of fresh 

 labradorite feldspar up to an inch in length. A diabasic texture 

 occurs in all but one dike, and such texture is sometimes very per- 

 fectly developed. In the table on page 31 the mineralogical com- 

 position of a sample from each dike is shown. The augite is the 

 common pale reddish-brown variety with good cleavage, and the 

 hornblende is the common green pleochroic variety, the chlorite 

 having been derived from either of these. 



The diabase iy2 miles northeast of Guideboard hill (no. 44 of 

 table) is of interest because it shows at once porphyritic and 

 diabasic textures and some glassy ground mass. Its weathered 

 surface has a distinct pitted appearance due to more rapid dis- 

 solving out of the feldspar phenocrysts. 



1 Since this was written the writer has discovered a rock of almost ex- 

 actly the same sort within the Blue Mountain quadrangle and which is 

 there very clearly only a metamorphosed portion of a gabbro stock. 



