56 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
which plug the steam-holes of amygdaloids:—the deductions to be drawn 
from the present investigation will show that the term employed, instead 
of covering, exposed that which it was intended to conceal. 
From the very frequent occurrence of difficultly-recognisable green minerals 
in volcanic rocks, it is to them that this term of “chloritic” is now specially 
applied ; it is so applied in the belief and hope that the mineral is a chlorite of 
some kind, and this is conceived to be quite a sufficient amount of knowledge. 
In proof that the case has, in the above, neither been misstated nor over- 
stated, I beg to refer to a paper, entitled ‘‘ Notes on the Occurrence of Chlorite 
among the Lower Silurian Volcanic Rocks of the English Lake District,” which 
was lately published in the “ Mineralogical Magazine,” by Mr J. Cutrron 
Warp, of Her Majesty’s Geological Survey. 
In this paper, which is a record of much careful observation, Mr Warp very 
seldom uses the term chloritic mineral, and commits himself to the substance 
being chlorite itself. In this belief he is so confident that he states that the 
chlorite can be seen to pass into magnesian mica, and even into potash mica; 
he discusses the nature, or rather the stages of the metamorphism which 
accomplishes this change ; and yet the paper not only does not state that any 
steps were taken to determine the actual nature of the substance treated of in 
this important speculation, but it bears internal evidence that neither analysis 
or any satisfactory test had been adopted for the determination. 
In this memoir the mineral in question is directly referred to upwards of 
thirty times, and indirectly several times more; and yet, 2 the evidence of 
Scotch rocks is not in absolute discordance with those of the English rocks, I 
am able to say that, while at three of the localities mentioned,” it is possible 
that the mineral may be chlorite, in all the rest it is certainly a substance 
which has no connection therewith. 
It is not meant to be insisted on that true metamorphism may not take 
place in volcanic rocks ; but a study of Mr Warp’s admirably precise descrip- 
tions goes to show that degradation, or at least not metamorphism pure and 
simple, had altered the rocks he treats of—had filled their vesicles, and lined 
their nests. And, while decomposition and recomposition through degradation 
is not insisted on as the sole possible change, it has to be said that it is the 
only one which has as yet been clearly shown to alter such rocks. 
But Mr Warp’s position, as he himself shows, is precisely that of the first 
petrologists of the day. He writes :—-“ Under the head of Diabase, ZIRKEL 
* Great Gable, Eskdale, and Harter Fell. The fan-shaped crystalline groups of Harter Fell are 
very probably chlorite. The nests from the same locality, ranging up to 1 inch in diameter, composed 
of a “fibrous crystalline” mineral, can hardly be so. As these large nests, a single one of which 
would suffice for an analysis, can, with “a little careful hammering, be taken out whole from the rock,” 
it is much to be regretted that they were not examined ; as a fibrous crystalline green mineral would 
in all probability prove to be either Kirwanite, or new. 
