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NEW YORK STATE MUSEUM 



The wider distribution of these dikes is of interest. The 

 syenite dikes do not range far to the west and south of Rand 

 hill, and so far as present knowledge goes must be counted by 

 tens in the Adirondack region as a whole. But the diabase dikes 

 range farther to the west and far to the south, and there are 

 hundreds of them. It is also of interest, taking a still wider 

 view, to note that igneous activity was widespread over the land 

 areas and shallow sea bottoms to the east and west of the Adiron- 

 dacks during these very ancient times, and that the Adirondack 

 eruptives are very like those which characterized Keweenawan 

 time in the upper lake region, and probably approach them as 

 nearly in age as they do in character. 



These dikes vary in width from an inch or two to forty or fifty 

 feet. Their contacts with the inclosing rocks are exceedingly 

 sharp. Joints, due in large part to shrinkage on cooling, are 

 much more numerous than in most of the adjacent rocks, so that 

 the dike rocks weather out into a multitude of sharply angular, 

 small blocks. 



The syenite dikes are usually of some shade of red, ranging to- 

 ward and passing into a brownish black in some instances. They 

 mostly show rather large, porphyritic feldspars and sometimes 

 black mica as well. The narrow dikes and the margins of the 

 larger ones consist of very fine grained, flinty rock. Their gen- 

 eral composition is of magnetite or specular hematite, hornblende, 

 biotite, microperthitic feldspar and quartz, but they vary greatly 

 in acidity and hence in the relative proportions in which these 

 minerals are present. Some of these dikes have been described 

 in considerable detail elsewhere, and those more recently dis- 

 covered add little that is new. 1 



The diabases are without exception black in color, weathering 

 to yellowish brown along the joint planes, so that the small 

 weathered blocks show simply this color. They vary in grain 

 with size, but all are very fine grained. Nearly all show porphy- 

 ritic feldspars which in a few dikes are notably large and may 

 reach a length of an inch, but which are usually minute. These 



Pushing, H. P. Bui. geol. soc. Amer. 9:239-56. 



