ADIRONDACK MAGNETIC IRON ORES 63 



much as a square mile. They are of special interest because they 

 contain bodies of low-grade titaniferous magnetite in numerous 

 localities. These ore deposits have received some practical atten- 

 tion and will be mentioned in detail later. 



The gabbros and their hornblendic derivatives may be very 

 easily confounded with the basic phases of the syenites to be next 

 described. The two rocks look extraordinarily alike. Yet under 

 the microscope at least, the syenitic varieties display abundant 

 orthoclase, the gabbros plagioclase. The garnets are more abun- 

 dant in the gabbros, although they do not entirely fail the syenites. 

 The granular fracture of magnetite is more in evidence in the 

 gabbros, but when all is said, the field observer may often be in 

 much doubt when confronted with the dark, basic gneisses, as to 

 which rock he is dealing with. For the present we may consider 

 them distinct. 



Augite syenites and related types, more or less gneissoid. The 

 syenites, now that they are well understood, are proving to be one 

 of the most important members among the rocks of the eastern 

 Adirondacks. They were first identified in the west and north 

 by C. H. Smyth jr, and H. P. Cushing, respectively, for although 

 gneissoid members with the corresponding mineralogy were found 

 in the east, they were at the outset placed with the doubtful 

 gneisses and were not recognized as distinct eruptives. The 

 diamond drill cores at Mineville have done much to clear up their 

 identity, and as they afford perfectly fresh rocks in definite 

 relationships, they are in the highest degree illuminating. For 

 several years they have been carefully saved and recorded by 

 Witherbee, Sherman & Co., and have been of the greatest service 

 in the preparation of this description. 



The syenite is an extremely variable magma which must have 

 been sharply differentiated into contrasted products, which then 

 constituted different layers in the fluid mass at the time of intru- 

 sion into the older rocks. At Mineville, diamond drill cores have 

 in one case been available showing a continuous section of nearly 

 1000 feet; in another of nearly 1400 feet, and in many others of 

 less, so that the relationships of the several layers can be carefully 

 studied. 



The typical syenite consists of microperthitic orthoclase — that 

 is of orthoclase filled with flattened spindles of albite — of emerald 

 green augite, which looks black in the hand specimen; of brown 

 hornblende, and of less abundant hypersthene. Magnetite is of 

 course present in subordinate amount, and titanite, apatite and 



