GEOLOGY OF THE NORTHERN ADIRONDACK REGION 311 



ience with the ordinary Adirondack anorthosites, white feldspar 

 is rare and where occurring is due to local alteration, which is 

 distinctly not the case in these " Whiteface " rocks. These are 

 also much richer in hornblende and pyroxene than the ordinary 

 auorthosites, though nearly pure feldspathic types have some 

 small local development, as for example near the bridge at 

 Franklin Falls. They differ also in the prevailing very gnei&soid 

 character, but occasional feldspar augen do occur and sometimes 

 reach considerable number and size. A single hand specimen of 

 the feldspathic rock from Franklin Falls which lies before me, 

 shows three such augen which are more than an inch in length, 

 besides several smaller ones. The structure is plainly cataclas- 

 tic in these less gneissoid types. 



The slides from the Clinton and Franklin county rocks show a 

 general predominance of hornblende over pyroxene, though both 

 occur in considerable amount. The pyroxene is a deep green 

 augite, no hypersthene having been noted. The feldspar is 

 mostly labradorite, as indicated by maximum extinctions of from 

 22° to 27° in the different slides. They show marked strain 

 phenomena, such as undulatory extinction, bent twinning lamel- 

 lae, and w r edge-shaped or pinched out twinning lamellae. There 

 is always some untwinned feldspar present, which is however 

 thought to be labradorite. The accompanying minerals are the 

 same as in the usual anorthosite, iron ores, zircon, apatite, 

 titanite, garnet, and sometimes a little quartz, usually as a by- 

 product of the garnet formation. The mineralogy of the rock, 

 the local cataclastic structure, and the fact that it occurs in a 

 considerable mass, surrounded on all sides by other rocks, seem 

 to point to its igneous nature. The apparent interbanding with 

 Grenville rocks at the edge of the mass gives that portion a 

 sedimentary look, but all the other igneous rocks of the district 

 show similar phenomena at their borders, and it would seem 

 that the clues to the origin of these rocks must be sought in the 

 least changed, most massive portions, rather than in their 

 peripheral phases where metamorphism has been most exces- 

 sive. The rock is therefore regarded as igneous and as belong- 

 ing to the later intrusives. Its localized distribution would 

 seem to indicate that it represents a separate intrusion rather 



