it T. S. Hunt on the Criticisms of Prof. Dana. 
wanting in unity of plan and purpose; and that parts of it, if 
we may hazard a conjecture, seem to have been written while 
he still inclined to the views of the opposite school. From the 
e of pseudomorphs which he has given, and from many 
passages in the text, it might be inferred that he then held the 
notions of Rose, Haidinger, ete., which he elsewhere, in the 
same paper, speaks of as being entirely different from his own. 
The views of Delesse, about this time, underwent a great change, 
which has a historic importance in connection with those which 
I advocate. When, in 1857 and 1858, he published the first 
and second parts of his admirable series of studies on metamor- 
phism, Delesse held, in common with nearly every geologist of 
the time, to the eruptive origin of serpentine and the related 
magnesian rocks. Serpentine was then classed by him with 
other ‘‘trappean rocks;’ and he elsewhere asserted that ‘“ granitic 
and trappean rocks” undergo in certain cases a change, near 
their contact with the enclosing rock, by which they lose silica, 
alumina and alkalies, and acquire magnesia and water, being 
thus changed into a magnesian silicate; which may take the 
form of saponite, serpentine, tale or chlorite. [Ann. des Mines, 
V, xii, 509; xiii, 398, 415]. It would be difficult to state more 
distinctly the view, which he then held, of the origin of these 
magnesian rocks and minerals by the chemical alteration ms 
ticed, in which, in place of the theory of epigenic pseudomor- 
feantion of various mineral silicates, taught 
by the German school, he brought forward, in explanation of 
the facts upon which this was based, another theory, which was 
only * extension of that already maintained by Scheerer and 
myself. 
It was not until 1861 that Delesse published the last part of 
his studies on metamorphism, which appeared in the Memoirs 
maintaining, as in 1858, that they are derived from the latter, 
Delesse, in 1861, asserts, on the contrary, that ‘the plutonic 
