GEOLOGY OF THE VICINITY OF LITTLE FALLS 21 



freshly broken surfaces will show at least patches of the green. 

 The grain is usually quite fine, but the quartz is coarser than the 

 other constituents and tends often to the ^' leaf " type, though 

 but rudely so. In places a few larger feldspars show also and 

 appear like small examples of porphyritic feldspars. The rock 

 is prevailingly of feldspar, and feldspar of an acid type. It is 

 quite quartzose, that mineral making from 15;^ to 20^ of the rock 

 on the average with a usual range of from lOfo to SO^. Pyroxenes 

 are the usual dark colored constituents, though both hornblende 

 and biotite mica also occur. Without going into detail at this 

 point suffice it to say that the rock has the precise mineralogy 

 of an augite syenite, the same mineralogy as the rock at Little 

 Falls and Middleville, and the same as the great mid-Adirondack 

 syenite masses. It differs from them in being in general some- 

 what more quartzose, in being everywhere thoroughly gneissoid, 

 and in the lack of porphyritic feldspars. It is however not to 

 be in any w^ay distinguished from some varieties of the rock in 

 the other exposures. On the other hand, it has nearly, if not 

 quite, as strong a resemblance to the greenish gneisses (3) just 

 previously described. 



Mixed rocks about the syenite. A large part of the pre-Cambrian 

 area of the sheet shows rocks which are very like the syenite rocks 

 and are thought to represent phases of them, but which are in- 

 extricably intermingled with smaller masses of undoubted Gren- 

 ville rocks. The syenitic rocks predominate, though they are not 

 of the normal type, but the Grenville rocks are in considerable 

 force, and the relations between the two are wholly obscure. It 

 was found impossible to separate the two in mapping on this 

 scale, and therefore the complex as a whole is given a separate 

 coloration on the map. Since the rocks seem to pass into the 

 syenites on the one hand, and into belts in which the Grenville 

 sediments preponderate on the other, the mapping of boundaries 

 must, however, be a wholly arbitrary matter. 



A very mixed lot of rocks is found in this belt as mapped, 

 and with frequent changes from one sort to another. The most 

 abundant type of all is a greenish gneiss, weathering brown. 



