1881.] 
261 
[Wadsworth. 
propylite. In certain instances the microscope revealed the pres- 
ence of minute grains of quartz, and the rock thus characterized 
came to be classed as quartz-propylite ; but there was never any 
doubt as to the generic nature of the rock. There was not a sol- 
itary instance in which the rock by us called propylite proved to 
be either diorite, andesite, or plagioclase hornblende-trachyte. 
I am careful to mention this fact, not as a guarantee of the cor- 
rectness of our determinations , for that has been placed beyond 
question by the microscopical analyses of Zirkel , but because 
later in this chapter I shall have occasion to discuss what consti- 
tutes a species of volcanic rock, and the factor which habitus 
must necessarily play in classification.” 
Now Professor Zirkel says (VI, p. 132) : “ Perhaps it may not be 
superfluous to insist that all the rocks described in the foregoing 
pages as propylites and andesites were first referred to one or the 
other group by geological observations in the field , and that the 
petrographic al diagnosis and the classification of . them have not 
been influenced by any artificial point of view or preconceived 
opinion. The examinations have proved that in every rock the 
geological and petro graphical differences perfectly accord .” The 
assertions in the above quotations do not seem to me to be sus- 
tained by the facts, as I think will ajztpear. 
Regarding King’s statements about himself or his assistants 
never wrongly assigning a rock to propylite in the field, and that 
the microscopic work always agreed with the field work, it is 
only necessary to quote their own statements. It will of course 
be admitted that they have not taken pains to afford contradic- 
tory evidence. 
Mr. King states in the same chapter, the italics as before being 
mine (I, p. 567) : 
“The field habit of this dacite is decidedly more propylitic than 
andesitic. ... in the field and in hand specimens we were 
often unable to distinguish between it and quartz-propylite. But 
in the case of this outburst it might readily be mistaken for the 
neighboring quartz-propylite .” 
Zirkel describes this rock as “ a dacite, which envelopes so 
many strange fragments of another variety of dacite as to form 
