98 
MUSEUM BULLETIN NO. 2. 
(2 of section, page 96) carries a fauna which has been referred 
to the Spence (Middle Cambrian) in the House range (see 
page 96) and which is to be correlated directly with No. 
21 of the Highland Range section. The Middle Cambrian 
faunas of the two shales bear more or less striking resemblances 
to each other. Gilbert, who was the first to describe the district, 1 
appears to have had little doubt as to the unity of the different 
shales (2 and 4 of section, page 96), and attempted to account 
by faulting for their presence near the mines. Pack’s descrip- 
tion 2 of the two formations (their occurrence and their meta- 
morphism, particularly the obliteration of bedding planes in both 
the quartzite and limestone, the pockety nature of the upper 
shale outcrops and their variation in thickness from 4 to 100 
feet in distances of half a mile, the fact that many of the ores 
of the district occur as “bedded deposits” along fault planes 
in the upper shale formation, etc.) also bears internal evidence 
of the possibility that the two formations may be the same. 
The relations between the two shale series and their correlation 
is further discussed in the section on the Pioche formation, pages 
120-125. 
Arizona, Grand Canyon , — The sediments overlying the Pre- 
Cambrian in the Grand Canyon region have been referred to 
the Tonto group and placed in the Middle Cambrian. 3 They 
are stated 4 to have been deposited upon an erosion surface 5 
transecting at least 13,000 feet of strata and cutting deeply 
into the Archaean. This pre-Tonto series consists of limestones 
and shales remarkable for their lack of metamorphism, but 
the evidence of an unconformity so profound as the one by 
which they are separated from the Cambrian leaves little 
question as to the correctness of their reference to the Pre- 
1 U. S. Geog. Surveys West 100th Meridian, vol. Ill, 1875, pp. 257-261. 
s Sohool of Mines Quarterly, Vol. XXVII, 1906, pp. 291-292 and 294-296. 
3 Walcott: Smithsonian Misc. Coll., vol. 53, No. 5, 1908, p. 167. 
* Walcott: American Jour. Sci., 3d. ser., vol. XXVI, 1883, p. 440. 
5 The unconformity at the base of the Cambrian involves erosion and the line 
will frequently be spoken of as a plane of erosion or an erosion surface. It is to be 
understood, however, that the Cambrian might be in contact with several or even 
all of the underlying Pre-Cambrian formations through depositional or other causes 
without postulating a period of erosion so intense as actually to have removed the 
missing strata. And such an explanation is favoured by the continental origin assigned 
to the Pre-Cambrian by such observers as Barrell (Jour. Geol., vol. 14, 1906, pp. 
553-568). 
