34 HORN EXPEDITION GENERAL GEOLOGY. 



still further increase at the expense of those coloured Pre-Canibrian in proportion 

 as these areas are subjected to more rigorous and searching examination. Accord- 

 ing to some of the highest authorities of the present day the Pre-Cambrian rocks 

 can often be divided into two groat groups, one including the rocks of eruptive 

 origin, the other those of sedimentary origin. In the first division are included 

 the gneisses, schists, and eruptive rocks, which uiiderlie uncomformably Cambrian 

 strata, or, where members of the second group are developed, either underlie them 

 unconformably or intrude them. In the latter case the gneisses are of later date 

 than the rocks of the second group. In the second group are included the rocks, 

 which can be proved to have had a sedimentary origin. Tliese usually overlie the 

 gneisses, etc., of the first group. As an example of such a classification wo may 

 take the two groups of rocks of Pre-Cambrian age, which are developed in North- 

 west Scotland, and known as the Lewisian gneiss and the Torridon sandstone. 

 The former underlies the latter, and the latter the Cambrian strata unconformably. 



In North America the Pre-Cambrian rocks have been divided into two great 

 groups corresponding .somewhat to those made out in Scotland. The lowest and 

 oldest, consisting of gneisses, mica, schists and granites, are included under the 

 name of "Fundamental Complex." This great group is overlain unconformably 

 by an enormous thickness of more or less highly altered sedimentary rocks, and 

 intercalated volcanic lavas and tufis. 



This latter American group, to which the name " Algonkian " has been 

 applied, has been again divided into three groups, each .separated from those above 

 and below by strong uncomformabilities. The total thickness of this latter division 

 has been estimated at G5,000 feet. 



In the uppermost strata of the Algonkian group distinct traces of fossils liave 

 been detected by Mr. Walcott.* 



In Brittany too. Dr. Barrois has discovered organic remains in the form of 

 Radiolaria in rocks of Pre-Cambrian age, which there consist of graphitic quartzitos. 

 At the meeting of the British Association for the Advancement of Science at 

 Oxford, August, 1894, the supposed organic nature of the bodies termed Radiolaria 

 by Dr. Barrois was called in question, the chief arguments against their organic 

 origin being based on their very minute size, their diameter being on the average 

 only about one-sixteenth that of the average diameter of recent Radiolaria. 



The Pre-Cambrian area of Central Australia area is, geologically speaking, 

 practically a terra ittcognita. Our visit to this inaccessible region extended over 



* Tenth Aiimial Report U.S. Ceol. Siuv., 1890, Walcott, p. 5.'i2. 



