146 GEOLOGICAL AND 



FIRST FORMED BASIC EL.3EOLITE-ZIRCON-SYENITE. 



Recent study of these elseolite-zircon-syenites has re- 

 vealed the fact that the oldest form is a thoroughly basic 

 rock of a greenish black color, quite porphyritic resem- 

 bling porphyritic diabase. Microscopical examination of 

 thin sections of this rock in polarized light shows that it is 

 composed of augite, green and brown hornblende, biotite, 

 plagioclase and {in abundance of titanite and rntile micro- 

 liths, micro- zircons and apatite. The porphyritic phigio- 

 clase crystals and also the hornblende areas are seen to 

 have numerous patches of elseolite and perhaps sodalite as 

 inclusions in them. The sodalite being isotropic and both 

 the minerals in the section, after treatment with hydro- 

 chloric acid and staining with fnchsine in water, show the 

 plngioclase and hornblende to contain numerous areas of 

 these minerals which gelatinize. Some of the elreolite in 

 these sections contains numerous feathery and fan-shaped 

 zeolites that are probably natrolite. These are displace- 

 ments of the decomposing elseolite. Everywhere on the 

 surface this decomposition of the eleeolite is seen changing 

 the color of this mineral from an oily green to a dull lead 

 color. The biotite is very fresh and of a red color and 

 granular masses of titanite surround grains of titan if- 

 erous magnetite, secondary products of this iron ore. 



As this rock mass does not contain olivine — olivine has 

 not been detected in any of the elseolite-syenite rocks of 

 this region — and as it is not found to occur in narrow veins 

 and dykes, it cannot, therefore, be attached to the Mon- 

 chiquit series as suggested by Prof. Rosenbusch (T. M. 

 M. M., xi, 1890, p. 447, Hunter and H. Rosenbusch). I 

 would, therefore, propose the name of Essexite for this 

 ancient eloeolite-zircon-syenite rock, which is probably the 

 tirst formed rock of the eheolite-syenite magma in this re- 



