276 T. S. Hunt—History of Pre-Cambrian Rocks 
having for its base a great mass of crystalline limestone, and 
consisting in addition to this of gneisses, generally hornblendic, 
and quartzites, interstratified with similar limestones. To thi 
series, as displayed north of the Ottawa, Logan assigned an ag- 
gregate thickness of over 17,000 feet, though the later meas- 
urements of Vennor, in the region south of the Ottawa, give to 
it a much greater volume. The geographical distribution of 
this limestone-bearing Grenville series gives probability to the 
suggestion of Vennor that it rests unconformably upon the 
basal Ottawa gneiss. 
These two divisions constitute what was designated by Logan, 
in his Geological Atlas, in 1865, the Lower Laurentian, the name 
of Upper Laurentian or Labradorian being then, for the first time 
given by him to aseries supposed to overlie unconformably the 
former, of which it had hitherto been regarded as constituting 
a part. This third division has already been referred to as 
characterized by the predominance of great bodies of gneissoid 
or granitoid rocks, composed chiefly of labradorite or relat 
anorthic feldspars, and apparently identical with the norites of 
Scandinavia. With these basic rocks are interstratified crystal- 
line limestones, quartzites and gneisses, all of which resemble 
those of the Grenville series. This upper group, for which the 
writer in 1871 proposed the name of Norian, was supposed by 
ig a to be not less than 10,000 feet thick. 
or farther details of the history of these various groups of 
pre-Cambrian rocks, and their distribution in North America, 
the reader is referred to a volume published in 1878 by the 
Second Geological Survey of Pennsylvania, being Part I of the 
writer's report on Azoic Rocks, intended as an historical intro- 
duction to the subject. 
Ill._—Tux History or Pre-Camprian Rocks 1n Great BRITAIN. 
In an address before this Association in 1871, in which the 
writer maintained the Huronian age of a portion of the ery® 
talline schists of New England and Quebec, he further e& 
pieted the opinion, based in part upon his examinations at 
olyhead in 1867, and in part upon the study of collections ™ 
London, that certain crystalline schists in North Wales would 
be found to belong to the Huronian series. The rocks 12 
question were by Sedgwick, in 1838, separated from the base 
of the Cambrian, as belonging to an older series, but wert 
subsequently, by Delabeche, Murchison and Ramsay, descr! 
and mapped oe sibeneh Cambrian strata, with associated intra 
sive syenites and feldspar-porphyries. 
In South Wales, at gt David's in Pembrokeshire, is another 
area of crystalline rocks, which the geological survey of Great 
