160 REPORT ON THE PETROLOGY OF THE DOLERITES 



and Tasmanian rocks. As there is nothing in the brief description cited above to suggest 

 any great excess of magnetite, some suspicion is thrown on the correctness of the figures 

 given for these oxides. Otherwise the analyses are not unlike those of Tasmanian rocks. 



In British Guiana, overlying the basement complex, there is an extensive series 

 of almost horizontally bedded sandstones, which continue across the borders into 

 Venezuela and Brazil. They are barren of fossils. In the last two countries there is 

 evidence for believing them to be of Cretaceous age, but C. B. Brown considers them to 

 be Triassic on the grounds of their great lithological resemblance to the sandstones of 

 that age in North America. They are intruded by dykes and sills of dolerite (" diabase "), 

 which are of Cretaceous or Tertiary age, and Harrison leans to the latter alternative.* 

 In microscopical features they are very similar to the Antarctic, and the comparison of 

 the analyses quoted (No. VI) of the rock from the Cuyuni River with that made by 

 Dr. Prior (No. Ill) will show how very similar are the two rocks in chemical composition. 



In this connection might also be noted the occurrence of quartz-dolerites in the 

 Mesozoic Newark system near New York, of which the famous Palisades on the Hudson 

 are a part.f Wahl has shown that enstatite-augite occurs in these rocks, and they have 

 other features analogous with those of the rocks here considered. 



The author's thanks are due to Professor David and Mr. R. E. Priestley for the 

 opportunity of examining these rocks, and to Mr. Allen Thomson for his assistance. 



Sydney 



December 1910 



* See J. B. Harrison, The Geology of the Gold fields of British Guiana, p. 23, and also chap. x. 



f See J. V. Lewis, " Palisade Diabase of New Jersey," Amer. Jour, of Science, vol. clxxvi, 1908, 

 p. 155 ; " Structure on Correlation of the Newark Trap Rocks of New Jersey," Bull. Geol. Soc. of 

 America, vol. xvii, pp. 195-210 ; " Origin and Relations of the Newark Rocks," Ann. Rept. State 

 Geologist of N.J. for 1906, pp. 97-129. 



EXPLANATION OF THE PLATE 



Figure 1. — Plagioclase crystal surrounded by quartz and felspar micropegmatite. 

 Erratic P. 363 from Cape Royds. Polarised light, magnified L45 diameters. 



Figure 2. — Intergrowth of a twinned augite crystal in enstatite. Erratic No. 93 from 

 Cape Royds. Polarised light, magnified 26 diameters. 



Figure 3. — Parallel growth of ophitic augite and enstatite. The darker central portion 

 of the pyroxene crystal is enstatite, the lighter surrounding portion augite, showing 

 basal striation. Erratic No. P. 82. Knob Head Mountain moraine. Polarised 

 light, magnified 26 diameters. 



Figure 4. — Essexite dolerite showing hypersthene and biotite, the latter in dark basal 

 sections and lighter vertical sections. Erratic from Cape Royds. Ordinary light, 

 magnified 26 diameters. 



Figure 5. — Hypocrystalline ophitic dolerite. Erratic No. 1100 from Cape Royds. 

 Ordinary light, magnified 26 diameters. 



Figure 6. — Aphanitic dolerite. Erratic No. P. 366 from Cape Royds. Ordinary light, 

 magnified 26 diameters. 



