566 



Report of the State Geologist. 



The sub-divisions of this dike bear in all directions, re-uniting and 

 forking again, so that several of the anorthosite members of the section 

 represent enormous horses in the diabase. 



Fourteen dikes have been found here within an area of less than a square 

 mile, and if all the branches were counted separately this number would be 

 trebled. The heavy second-growth on Rand hill undoubtedly conceals many 

 more. The only two of these dikes that call for any especial mention are the 

 two bostonites.* No. 7 is of the red, aphanitic type, with numerous small 

 porphyritic orthoclases showing. It contains a little magnetite and chlorite, 

 and the now structure of the ground-mass is well marked. No. 9 represents 

 the other extreme of these rocks. It is coarser, the whole appearing non- 

 crystalline to the eye. It is much darker in shade, due to a large biotite con- 

 tent. Both orthoclase and biotite occur in two generations, the orthoclase 

 phenocrysts reaching a length of over one centimeter. The ground-mass is 

 quite coarse, and contains apatite and augite in addition to the orthoclase 

 and biotite. Quartz has not been recognized ; if present it is only sparingly 

 so. This is the only dike of this type so far noted in the region which has 

 phenocrysts of a ferro-magnesian mineral, and the great abundance of the 

 biotite in this dike is quite unusual. 



Chazy. 



No Pre-Cambrian rocks appear in Chazy township. 



Series IV. Potsdam sandstone. The Potsdam occupies the north- 

 western half of the township. That on the extreme west belongs to the high 

 plain and pertains to the basal part of the formation. That on the low- 

 grounds, on the contrary, is toward the summit, the passage beds to the Cal- 

 ciferous appearing just over the border in Champlain township. A great 

 fault separates the high level from the lower Potsdam, and another great 

 fault lies between the Potsdam and the Ordovician limestones to the east of 

 it. Most of the rock is of the ordinary character, and calls for no comment. 

 A curious black layer, several feet thick, is exposed in a railroad cut just north 

 of Chazy depot. While mainly a quartz rock, occasional grains of orthoclase, 

 microcline, plagioclase, hornblende and magnetite occur, and large zircon 

 grains are a feature. The quartz is full of minute black inclusions, and 

 there is also a black cement of uncertain nature to which the color of the 

 lock is due. 



* Trachyte* would be a better term us that is what the rock is, and no useful purpose is served by the use of the other so far 

 as the writer can see. It is used here merely to make the nomenclature conform to that already in use in the region. 



