284 BULLETIN OF THE 
those geologists who hold that conglomerates have been metamorphosed 
into felsites, have forgotten that conglomerates are usually mixed rocks, 
while in the cases cited by them the pebbles are felsites, the same as 
the felsite into which they are said to be changed. We should expect, 
when an outflow takes place, that there would be a re-heating and 
breaking up of the old lava in the vent, which would naturally give us 
this brecciated material. However old the rocks may be, the materials 
of the first class in each species remain the same in character, from the 
earliest ones that I have examined down to those of the present time. 
The alterations to which volcanic rocks and their derived sediments 
are subjected make it often difficult to distinguish one from the other ; 
but this difficulty arises, not from the sedimentary rock assuming, under 
any agency to which it is naturally subjected, the form of a volcanic 
one, except so far as it retains the characters and alterations belonging 
to its parent rock, but from the volcanic one taking from alteration, 
characters closely like those of sedimentary rocks. I have never found, 
under the microscope, the sedimentary characters obliterated in the 
most highly metamorphosed rocks that were known to have a sedi- 
mentary origin; and, furthermore, the minerals and mineral varieties 
of the third class in sedimentary rocks are, with possibly one or two 
exceptions, never like the minerals arising from the crystallization of 
any lava. The supposed passage of a sedimentary rock into an eruptive 
one can only be considered proved when it is shown positively that the 
observer knows the nature of the rocks in question, and that he has 
examined every inch of the portion lying between them, so as to be able 
absolutely to prove that no line of junction can exist. One such case 
worked in that way would prove the entire question in favor of the 
popular geological school. 
The Geological Exploration of the Fortieth Parallel adopted, with mod- 
ifications, Richthofen's classification of the Tertiary and recent volcanic 
rocks, and Mr. King, in his report, states that each of these rocks shows 
always a basic, mean, and acidic form. He classes rhyolite as the acidic 
form of basalt, placing both under the head of neolite. As my exami- 
nation of Mr. King’s collections convinces me that his system is based 
on errors, it is well to point out briefly some of these, taking, of course, 
the report of Professor Zirkel as the basis, and following, in a measure, 
its order. 
The mica schist that Professor Zirkel calls a paragonite slate, like 
that from St. Gotthard, is similar to many mica schists in New England, 
and, except that its color is grayish-white, has no resemblance to the 
