REPORT OF THE DIRECTOR AND STATE GEOLOGIST r5l 



composition it is either a quartz-orthoclase-hornblende, or quartz- 

 orthoclase-augite rock. 



Under the microscope the same indefinite, granular structure 

 is shown which the ordinary gneiss possesses, except that near 

 the gabbro there are also corrosion zone (reaction rim) structures 

 quite like those in the gabbro itself. Along Mead brook are a 

 series of most excellent exposures of rapidly alternating gneiss 

 and gabbro, the latter cutting across the foliation of and sending 

 dikes into the former. This gneiss is essentially made up of 

 orthoclase, hornblende and quartz, with accessory zircon, apatite, 

 pyrite, magnetite, garnet and biotite. The magnetite is nearly 

 always surrounded by hornblende. The hornblende is patchy, 

 very irregularly bounded, and full of small biotite shreds which 

 eeein secondary; commonly also the magnetite and hornblende 

 are separated by a narrow zone of biotite, consisting of closely 

 packed, very minute shreds. Considerable garnet has formed in 

 places at the contact between hornblende and feldspar, accom- 

 panied by small grains of secondary quartz. Precisely similar 

 corrosion zones are found in the- gabbro near the contact. The 

 garnet zones are found all through the gabbro, but the biotite 

 zones have not been noted except near the contact, though they 

 may occur. The writer is disposed to explain the facts by the 

 assumption that the gneisses on the east and the west side of 

 the hill are the same, and that the differences noted on the east 

 side are due to contact metamorphism, with the production of 

 a narrow zone of mixed composition between the two rocks, due 

 to incorporation in the gabbro of material from the gneiss. 



The augite gneiss from the same locality is an even more flinty 

 rock, and in it garnet has been largely developed at the contacts 

 between both magnetite and augite, and feldspar. This would 

 imply that the feldspar is plagioclase, though it is largely un- 

 twinned. In the ordinary Dannemora gneiss garnet is a rare 

 mineral, though abundant in the Grenville gneisses. No similar 

 rock has been observed on the west side of the hill, and the 

 amount of change produced by the intrusion is a matter of un- 

 certainty. 



