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PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 551 
tion our inquiry must take is as regards any evidence the analyses may afford, 
bearing upon the union of the two species into one. 
It may be at once conceded that this question cannot be decided by any 
merely chemical evidence; the wide range of composition, hardly referable to 
anything closer than a general formula in either mineral, precludes such a hope. 
The little information that we do obtain tends in the direction of the separa- 
tion of the two ; it consists in this, that in what may be designated as “ recog- 
nised individuals” of the hornblendic type we have here and again a distinct 
replacement of silica by alumina ; and this we have not in recognised individuals 
of the augitic type. 
Passing from this to the lithological information, we find that a consideration 
of the records above made by no means tends to strengthen certain of the lines 
of demarcation which have been drawn between them. 
Put in a general form, two of these lines are thus defined: “These two 
minerals occur in distinct geognostic positions. Hornblende in rocks containing 
quartz or free silica, and mostly with minerals that are neutral compounds of 
silica, as orthoclase and albite. Augite in rocks that do not contain free silica, 
and mostly with minerals that are not neutral silicates, as labradorite, olivine, 
and leucite. 
“ Hence, there are two distinct series of igneous rocks: the hornblende series, 
including granite, syenite, diorite, &c. ; and the augite series, or hypersthene 
tock, gabbro, dolerite, &c. 
“Tn some rare instances these two minerals have been found together, either 
regularly conjoined or in distinct conditions.” * 
Here we have it clearly laid down that two “distinct series of igneous rocks” 
are established on the specific distinction between the two minerals; and 
secondly, that one argument for these minerals being themselves distinct is to 
be drawn from the fact that they are associated in but “rare instances.” 
It would appear at first sight to be an easy matter to enter at least upon 
the consideration of questions couched in language so precise as the above; 
but it proves to be not altogether so. 
We have it jist laid down here that hornblende occurs in rocks with free 
silica, and along with orthoclase and albite. 
Secondly, that augite occurs in rocks devoid of free silica, and,—in contra- 
distinction to hornblende,—along with labradorite, olivine, &c. 
Thirdly, that the hornblendic series of rocks,—granite, syenite, diorite,— 
are “distinct ” from the augitic series,—hypersthene rock, gabbro, dolerite, &c., 
Fourtly, that the two minerals, on the distinctiveness of which the above 
Separations have been made, occur together in rare instances. 
* Nicot’s “ Manual of Mineralogy,” p. 208. An epitomy of the views of Gustav Ros. 
