2S2 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 Neck, 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 localit}-, 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. 



It is found in those rocks which show signs of long-continued cooling 

 that the base diminishes and th§ crystalline minerals increase, the base 

 occupying the interspaces between the 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. Dillor, who lias Leen studying lithology under my direction, has xmdertaken, 

 as a subject for his thesis, the field and mieroscopic relations of some of the felsites of 

 Eastern Massachusetts. lie has already made some important observations, and I hope 

 his woik will solve the vexed question of their relations. 



