HORN EXPEDITION — GENERAL GEOLOCY. 33 



Foundation Eogks, previously called Pio-Silurian. Glen Helen Series (Oamliiinn 

 an<l Pre-Caiubiiau ?) is introduced l)y a quartzite, ()\erlaiii by crystalline lime 

 stones, dolomites, tlay-slate, etc.; rising through the eroded Lower Silurian 

 anticlines. Maueeno ]5luff .Series (Lower Silurian part) enihivices ilie t'ossil- 

 iferous limestones and associated sandstone and quartzites. Walker C-rioek Series 

 (Devonian'?) ; as previously, the author regards these Ijeds as unconforinahh^ 

 with the Ordivician, whereas we recognise them as only the more gently 

 undulating Larapintine sandstones (Ordovician). 



IT. — Pre-Cambpian. 



(a) Introduction. 



Much attention has of late years been paid ])y geolot;ists in (Ireat 

 Britain, on the Continent of Europe, and in North America to the study of 

 the important but very complex .series of rocks variously known as Arcluean, 

 Azoic, Pre-Cand)i-ian, etc., which underlie unconformaJily the base of the Olcncllus- 

 zone of the Cambrian system wherevei' it is developed. The result of this laborious 

 work has been the establishment of our kn(nvledge of the origin, tectonic structures, 

 and geological i-elations of tlie different members of this heterogeneous division of 

 rocks, and their relation to the overlying Paheo/.oic fossilifei'ous stra.t.i on a, sounder 

 basis. Unfoitunately very little work of a similar nature lias lieen done in 

 Australia, wliere tlu^ extensive and easily-accessible ai-eas of Prc-(Jandiiiaii rocks 

 of Yorke's Peninsul.a and the Mount Lofty i.'ange otTer special facilities for such a 

 study. Not many years ago the majority of the g(>ologists were incline<l to group 

 all rockonasses that had been subjecte(l to region.al metamorphism, .and had taken 

 on chai'acters of a certain natui-e in one class and include them all under one name, 

 such as that of Aicluean. 



This .system was supposed to include the oiaginal crust and ;dl sedimentary 

 strata deposited by the water of the ocean befor(> it had cooled down suHiciently 

 to permit life to appear in it. Ly others they were; considered to repicsent 

 originally sedimentary rocks comparable to the later fossiliferous strata, which 

 had been subjected to enormous compression, undergoing thereby great physical 

 and chenucal changes. In many cases the planes of foliation and clea\ag(^ w<>rc 

 interpreted as planes of stratification. These views have undei-gone considei'able 

 modification. Tn the first place many areas of metamoi'phic rocks ha\e been 

 relegated to their proper positions in the geological record as highly altei-ed 

 Palaeozoic, INIesozoic, and even Tertiary sediments. Tt is furthermore l)elie\ed that 

 the areas coloured on the geological maps as Pala'oz()ic, Meso/.oic, and Tertiary will 



