37 
anticline, and are surrounded by a thick accumulation of tuffs, agglomer- 
ates, and flows, which cover a very considerable area west of Harrison lake 
and occupy most of Echo island. The total thickness is about 9,000 feet. 
Fossils were obtained from a band of aqueous sediments in the lower part 
of the formation (locality No. 3, See Figure 1) and also from higher hori- 
zons. These show the volcanic assemblage to be of Middle Jurassic age. 
To the north, near Deer creek, to the east on Echo island, and to the south 
on Harrison river, the volcanics are overlain conformably by well-stratified 
tuffs. These have an average thickness of 2,700 feet. Only a few fossils were 
found in them and none of these is indicative of exact dates. However, the 
formation is overlain conformably by dark grey argillites, the upper part of 
which yielded excellent fossils of early Upper Jurassic age. This deposit of 
argillite, which is about 2,400 feet thick, is well exposed from the mouth of 
Deer creek across the hills to the northwest, through the valley of Myster- 
ious creek, and into the Chehalis Creek basin; also in another strip on the 
south side of Harrison river. In both sections the argillites are overlain 
by an accumulation of well-stratified tuffs that on Mysterious creek are 
about 1,800 feet thick. They have yielded early Upper Jurassic fossils. 
The early Upper Jurassic argillites and tuffs on the south side of Har- 
rison river are overlain unconformably by the rocks to which Bowen applied 
the name Agassiz series. The lower part of this series consists of about 
3.000 feet of unfossiliferous conglomerates. These are overlain by about 
5.000 feet of black argillites which contain poorly preserved, unidentifiable 
fossils. At this point the sequence is broken by the thrust faults. How- 
ever, presumably the same formation occurs as narrow strips on the 
west shore of the peninsula where it yields the Argovian ammonite. Ana- 
cardiocerds. 
The argillites that on the peninsula are regarded as part of the Agassiz 
series and the stratified Upper Jurassic tuffs near Deer creek, are overlain 
unconformably by early Cretaceous deposits bearing abundant fossils, 
mostly of the genus Aucella. The lower part of the Cretaceous series 
consists of a basal conglomerate overlain by grey sandstones and totalling 
1,260 feet in thickness. The upper part is made up of 2,200 feet of pyro- 
clastics overlain by about 1,500 feet of sandstones composed of waste 
from the volcanics. These Cretaceous beds occur in a strip running north- 
west from the peninsula, along the west side of Long island, to Broken- 
back hill and beyond. This strip is cut off along its east side by the thrust 
fault. 
The sedimentary rocks of the district are cut by plutonics thought to 
be of two ages. Those of one age are mostly altered quartz monzonite 
and occur in masses of various sizes, the largest of which is a batholith 
lying west of Chehalis creek. It intrudes the Middle Jurassic rocks and 
appears to have supplied pebbles to the basal conglomerate of the early 
Cretaceous. It is placed tentatively as late Jurassic. 
The other plutonic intrusive is a fresh quartz diorite. It occurs in 
stocks in various parts of the area, notably between Agassiz and Harrison 
Hotsprings, and at Doctor point, also in a batholith lying mostly in the 
upper part of the Chehalis drainage basin. These quartz diorites cut 
both Jurassic and Cretaceous strata and intersect the great overthrusts 
