Vict. Nat. 



QO Armitage, Country about West Essendon. [ ''^ggpt 



that the presence of granite in the neighbourhood is probable 

 Such sands occur south-east of Black Rock." 



The origin of the clay stratum and pebbles in the Sand Pits 

 has just been discussed. The nature of the sands and grits in 

 the loosely consolidated beds in the Pits, and microscopical ex- 

 amination of the grains in the quartzite, show that they are 

 undoubtedly of granitic origin. The nearest granites to West 

 Essendon now outcropping occur five or six miles to the north, 

 at Mount Gellibrand, Broadmeadows, and at Bulla. These 

 granites, w^hich emerge above the basalts that surround them, 

 show evidences of considerable denudation. From a southern 

 extension of these, now buried under the lavas of the plains, 

 the materials for the major portion of the Tertiary sediments 

 about West Essendon have undoubtedly been derived. 



The Tertiary Quartzite and Greywacke. 



At a distance of lOO yards or so to the south of the Sand Pits 

 there is a change from the loosely compacted, friable, ferruginous, 

 quartz sand deposits to intensely indurated quartzite, which 

 rings under the hammer. It palpably contains the same kind 

 of original material as occurs in the Sand Pits, with the addition 

 of a siliceous cement. At a short distance to the north of the 

 Sand Pits, beds somewhat similar to this quartzite, but of very 

 much finer ingredients, are met with. These have been regarded 

 as porcellanite. According to Sir A. Geikie (ii), " porcellanite 

 (argillite) is a name applied to the exceedingly indurated, some- 

 times partially fused condition, which shales are apt to assume 

 in contact with dykes and intrusive sheets or bosses. . . . 

 It is hardly possible to discriminate between such highly baked 

 shales and some of the fine siliceous sediments." The same 

 ' writer defines greywacke as " a compact aggregate of rounded 

 or subangular quartz, felspar, slate, or other minerals of rock, 

 cemented by a paste which is usually siliceous, but may be 

 argillaceous, felspathic, calcareous, or anthracitic." The litho- 

 logical character of the so-called " porcellanite " of the Horse- 

 shoe Bend does not accord with its being " a highly baked shale." 

 Its composition is such that it agrees closely with Geikie's 

 definition of greywacke (see Petrographical Note No. 3). 



In reference to the formation of quartzite, Mr. C. R. Van 

 Hise (12) says : — " One of the most important rocks produced 

 by the cementation process is quartzite. The term ' quartzite ' 

 is restricted to quartzose sand rocks which have been so firmly 

 indurated by the cementing processes that when broken the 

 fractures pass through the original grains and not around them." 

 With regard to the question as to whether quartzites are formed 

 below or above the sea, he writes : — " The cementation of sands 

 must take place after they have been raised above the sea, and 



