REPORT OF THE DIRECTOR AND STATE GEOLOGIST 



rGl 



formation of the inclosing rocks and of these later ones is argued. 

 It is thought that the amount of material removed during this 

 time will not fall short of the entire amount which has since been 

 removed, but, even if this be an overestimate, the intervening 

 period must have been long. 



As has been already stated, these rocks are found at the 

 present time only in dikes. We see the channels through which 

 the material ascended, but can not tell whether any reached the 

 land surface of that time, nor do we anywhere get a glimpse of 

 the reservoirs which supplied the material, since erosion has not 

 yet cut deeply enough to disclose them. The comparatively small 

 area now occupied by these dikes thus gives little idea of the 

 possible importance of this eruptive period. It seems however 

 improbable that great surface flows should have taken place, else 

 we should surely find remnants of them in protected places. It 

 seems more likely that the forces of eruption were spent in form- 

 ing intrusive masses from which the dikes radiated upward, 

 dying out above. The dikes plainly owe their existence to the 

 same causes which were responsible for the earlier, great intru- 

 sions, and mark the last paroxysm of igneous activity from that 

 source. 



The two distinct dike rocks of this period have been already 

 noted. On Rand hill the two are more nearly of equal import- 

 ance and abundance than in any other spot in the entire Adiron- 

 dack region, and dikes of both sorts are very numerous. 19 of 

 the red, syenite dikes have been found on the hill, of which seven 

 were noted in a previous report. 1 The black, diabase dikes are 

 even more numerous, but impossible to enumerate, since they are 

 frequently found combined into a great dike plexus of many 

 branches. The display of dikes is the most impressive known in 

 the Adirondacks. It must either signify an intrusive mass not 

 far beneath or else mark the spot whence much material was 

 poured out on the old surface above. 



x 15th an. rep't of the N. Y. state geol. 1895. p. 562-66. When this 

 report was written these dikes had not been differentiated from the 

 similar, post-TJtica dikes along Lake Champlain, and they are therefore 

 spoken of as bostonites. 



