282 BULLETIN OF THE 
a fact which is doubtless due to their base being so siliceous, This 
structure and their devitrification enable us to trace a direct connec- 
tion between the rhyolites and felsites, which are simply the older and 
more altered rhyolites. One of the best illustrations of this is to be 
found on Marblehead Neck, Mass., where at least two distinct flows of 
felsite occur, one cutting the other. They show the fluidal structure so 
characteristic of rhyolites, —a character that has been mistaken for lines 
of sedimentation by geologists, while the enclosed crystals of orthoclase 
have been taken for pebbles. 
These felsites are not stratified, and, contrary to the positive assertion 
of Dr. T. Sterry Hunt, they are younger than the granite on the N eck, and 
cut it ; also dikes of the felsite are seen cutting the granite. Furthermore, 
there is no passage of a conglomerate into a felsite in this locality, as 
macroscopic and microscopic observations prove. While to the naked 
eye and under the microscope this rock shows the fluidal structure of a 
rhyolite, in polarized light it is seen that the base has been completely 
devitrified, a process that is carried to a great extent in many known 
modern rhyolites.* 
The various old rocks, designated by a multiplicity of names, fall into 
one and sometimes two or more of these species, according to the exac- 
titude with which the name has been used in the past. The derived 
rocks can be referred to the parent ones with greater or less difficulty 
according as they were derived from one or more species, and also ac- 
cording to the changes that they have suffered. The tracing out of the 
natural relations and history of rocks is the most valuable and philo- 
sophical work to be done at present in this department of study, — work 
which never can be finished until the sum of human knowledge is com- 
plete, but which, as far as lies in my power to do with the collections 
now at my disposal, I hope to place before the public next year. 
Itis found in those rocks which show signs of long-continued cooling 
that the base diminishes and the orystalline minerals increase, the base 
occupying the interspaces between tho crystals. When this process was 
carried to sufficient extent, as it was in the deeper-seated and older 
rocks, the chemical bases crystallized out of the rock base, leaving the 
superfluous silica in the form of quartz occupying the same interspaces. 
This process has taken place not only in the acidic granites but also 
* Mr, J.S. Diller, who has been studying lithology under my direction, has undertaken, 
as a subject for his thesis, the field and microscopic relations of some of the felsites of 
Eastern Massachusetts. He has already made some important observations, and I hope 
his work will solve the vexed question of their relations. 
