PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 57 
remarks (‘ Microskopische Beschaffenheit,’ p. 407), that the diffused greenish 
mineral seems to be chlorite, and probably a decomposition product of augite, 
and the same is very likely the case among the Cumberland rocks, for the larger 
augite crystals, which occur in considerable abundance among some of the lava- 
flows, are often replaced by,” &c. Now, here surely is evidence of a sufficiency 
being obtainable for analysis. The eye can be educated to such a use of the 
microscope as to be able to trace a gradual process of decomposition, but never 
to determine the composition of an unknown mineral. 
Cotta also isin much the same position. In defining diabase, and enumerat- 
ing its components, he says, “and some chlorite.” ‘The green colour of the 
rock is chiefly owing to its chlorite.” ‘The most marked feature of its differ- 
ence from dolerite is its chlorite, and its consequent green colour. If this 
chlorite be a product of transmutation, then all the original difference between 
diabase and dolerite probably consists in the level or depth of solidification.” 
It is significant that CotTa does not enumerate chlorite among the minerals 
accessory to diabase in its clefts and veins; while he says, ‘The vesicular 
cavities are filled with ..... chlorite, glauconite, ..... and the like.” It is 
well known that the veins of a rock contain the materials of its general structure 
in giant-development of crystals, while its amygdules and druses contain the 
products of the transmutation thereof, through aqueous change and transfer. 
Lately, however, clearer light is beginning to be thrown on the matter, for 
we have Dana supporting RosensuscH when he says that chlorite is not an 
essential characteristic of diabase ; and those who look in Scotland for chlorite 
in rocks in all other respects entitled to the name of diabase will certainly find 
none. 
If we are to accept and found upon the above admissions—namely, that the 
amygdules of these so-called chloritiferous-volcanics contain the material which, 
diffused in minute granular or flaky condition throughout their mass, imparts 
the occasional green tint, then my analyses have determined that material to 
be in no way connected with chlorite except in colour. 
The results of the following investigation go to show that the minerals 
usually thrown together under the term which I somewhat unwillingly adopted 
as the heading of this chapter, are to be grouped under two absolutely distinct 
heads. For the first the old term of the chlorites may be retained ; to the 
second the term the saponites may be applied, from the unctuous feeling which 
is a marked characteristic of them all. 
So far as the evidence of over fifty analyses and much contingent observation 
entitles me to speak, the members of the first of these groups are found only 
in sedimentary and metamorphic rocks, never occurring in volcanics; the 
second are confined invariably to the latter, never being seen in the former 
class of rocks. 
