ADIRONDACK MAGNETIC IRON ORES \J 



limestone within the area, probably entangled masses borne up 

 0:1 the surface of the intrusion, but in the main the anorthosite 

 is unbroken by other rocks. Small outlying exposures of anor- 

 thosite have beeen found on the northern and southern borders, 

 the most remote being the Rand hill intrusion near Dannemora, 

 Clinton co., and the one near Bakers Mills, Warren co., both of 

 which lie some 20 miles distant from the proximate portion of 

 the principal area. 



In its normal development the anorthosite consists of little else 

 than feldspar which is generally a blue labradorite. This mineral 

 occurs as a rule in large interlocking crystals, giving the rock a 

 very coarse texture like that of porphyritic granites. The acces- 

 sory minerals include augite, hypersthene, hornblende, ilmenite 

 and magnetite. While usually constituting a small percentage of 

 the rock, the ferromagnesian silicates may assume such importance 

 as to mark a gradation toward or even a complete transition into 

 the gabbros. With the increase in the proportion of these min- 

 erals, there is also a change in the texture, which becomes finer 

 by diminishing the size of the feldspar and shows the characteristic 

 mottled aspect of gabbroic rocks. There are innumerable places 

 within the area where this variation is to be found. The gabbro 

 type, however, falls far short of the wide distribution of the felds- 

 pathic phase, being limited to patches and dikelike bands in the 

 latter. By compression the anorthosite has become laminated, 

 specially in the bordering zones where it often shows a thoroughly 

 gneissoid appearance. The feldspar crushes down to a white mass 

 of granules, in which remnants of the original blue feldspar may 

 usually be seen. The granulation is accompanied by the develop- 

 ment of garnet in the form of pink crystals surrounding the dark 

 silicates. 



Gabbro. This rock stands in close relation to the anorthosite. 

 It is abundant only within the area occupied by the latter or in 

 close proximity thereto. The gabbro is a black, very dense aggre- 

 gate of labradorite, augite, hypersthene and ilmenite or magnetite, 

 with olivine as a somewhat uncommon constituent. It seems to 

 have been a later differentiation of the magma which has given 

 rise to the anorthosite as it sometimes cuts the latter intrusively. 

 The gabbro inclusions which grade into the anorthosite are, how- 

 ever, contemporaneous segregations. 



The limited masses of gabbro have sometimes been so thoroughly 

 metamorphosed as to assume the character of amphibolites. The 

 presence of unchanged pyroxene, and basic plaoioclase feldspar 



