10 THE GEOLOGIST'S TRAVELING HA^D-BOOK. 



I. EOZOIC, (ARCH/EAN, AZOIC.) 



I. PRIMARY OR CRYSTALLINE ROCKS. 



The late investigations of American geologists have enabled them to establish 

 several divisions in the crystalline stratified rocks, which were originally called 

 Primary or Primitive. The name Azoic, formerly given to the Primary rocks to 

 distinguish them from the Paleozoic formations, has, since the discovery of Eozoon 

 in the former, been exchanged for that of Eozoic. The designation Archaean or 

 ancient rocks, is used by Professor Dana and others, and applies to the Primitive 

 formations without distinction. Among those who have made the Primitive or 

 crystalline rocks a special subject of study for many years, no one is more eminent 

 than Dr. T. Sterry Hunt, and as no proper account of the four groups into 

 which he divides them ; namely, 1 a. Laurentian, 1 b. Norian, 1 c. Huronian, 

 and 1 d. Montalban, has yet appeared, the following descriptions, which he has 

 kindly furnished for this work, are a very valuable contribution to the science of 

 geology. 



1 a. Uaurentian, The name of Laurentian was given in 1854, by the geological 

 survey of Canada, to the ancient crystalline terrane which forms the chief portion 

 of the Laurentide hills of Canada, and the Adirondacks of Northern New York. 



Throughout these areas the prevailing rock is a strong, massive gneiss, reddish or 

 grayish in color, sparingly micaceous, but very often hornblendic. The predominance 

 of this mineral occasionally gives rise to a nearly pure hornblende-rock, sometimes 

 with a little intermixed feldspar. The gneisses are, for the most part, distinctly 

 stratified, but occasionally the evidences of stratification are not very apparent, so 

 that these rocks have often been designated granites. This series is distinguished 

 by the absence of chloritic, talcose, argillaceous or micaceous schists. It includes, 

 however, crystalline limestones, of which there are supposed to exist, on the 

 Ottawa, three distinct formations in the Laurentian series, each of which is, in 

 parts, according to Logan, more than 1000 feet in thickness. These limestones, 

 which are generally coarsely crystalline, are often magnesian, and abound in 

 foreign minerals, chief among which are serpentine, chondrodite, hornblende, 

 pyroxene, magnesian mica, apatite and graphite. All of these occur both 

 disseminated in the beds, and, aggregated with other minerals, in veins, or 

 endogeneous masses. Associated with these limestones are often considerable 

 beds of quartz-rock, sometimes garnetif erous. Great masses of magnetic oxide of 

 iron are also found interstratified in this series. The measured thickness of the 

 Laurentian gneisses, with their included limestones and other rocks, on the Ottawa, 

 where the strata are nearly vertical in attitude, has been estimated at over 17,000 

 feet. Beneath these, known as the Grenville series, there is a great mass of 

 granitoid gneiss, without limestones, and of undetermined thickness, called the 

 Ottawa gneiss, which, it is conjectured, may not be conformable with the upper 

 portions, but is, as yet, included in the Laurentian series. 



