GENERAL MEETING. 79 
existence, prior to their denudation, of volcanic extravasations 
which probably covered them wholly or in part. In the mesa walls 
and on their slopes may be seen numerous instances of partially 
excavated necks, while in others the necks are just beginning to be 
exhumed. Im the latter cases remnants of the old cinder-cones 
which were piled up over their summits are still preserved, so that 
natural sections of the whole apparatus are exhibited. There are 
many scores of these necks, and the effects of erosion in unearthing 
them are exhibited in all stages. Wherever the true neck or core 
is disclosed the basalt is seen to be columnar, and the columns are 
often arranged in beautiful fashions. 
No more striking illustration and proof of a great erosion could 
be mentioned than is here disclosed, and the region must become 
a classic one, to be referred to by future geologists as an excellent 
example of some of the grandest laws and’ processes with which 
their science deals. 
Mr. PowE.u spoke of the distribution of eruptions. They are 
apt to occur on the faces of acclivities undergoing erosion, but not 
on acclivities due to displacement. Near a fault they break through 
the uplifted block rather than the thrown. They do not occur in 
the bottoms of cajions. . 
In mapping the Plateaus he had thrown the boundary farther 
north than Captain Dutton, so as to include a large area north of 
the Uinta Mountains. 
The peculiarly favorable conditions under which geology is 
studied in the plateau region enable its features to be comprehended 
without the doubts and the laborious compilation of details else- 
where necessary. It results that while the structure of the Plateau 
country is as well known as that of any equal area in the world, 
the literature of its geology is exceedingly small. 
Other remarks were made by Messrs. WHITE and GILBERT. 
258tH MEETING. NovEMBER 22, 1884. 
The President in the Chair. 
Forty-nine members and guests present. 
