
PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 207 . 
Seeing, then, that the treatment, from the first dusting of the powdered 
mineral through the finest: linen, to the final weighing of the insoluble residue, 
was throughout, as far as possible in the course of continuous laboratory work, 
identical, I can only say with Mr Wautace Youn that I “have not as yet been 
able to find out why the silicic acid should be so soluble in some cases and not 
_ so'in others, when subsequent examination showed it to be equally,” or nearly 
* equally, pure in all. | 
_ A perfectly new field of research lies open here. 
The Selection and Purvfication of Specimens for Analyses.—This is not only 
of the first importance, but it is all-important. It is evident that it is better to, 
refrain altogether from analysing than to operate on material which has not 
_ been purified as far as it is possible for our appliances and our patience to 
effect. It is not altogether unusual for those petrologists, who within the last 
few years have been availing themselves of the aid lent by microscope in. the 
- examinations of sections of rocks, to disparage the labours of the analyst by 
the assertion that he works upon material which is visibly imptre, and that his 
results should therefore be disregarded. Very fittingly might the chemist reply, 
that the knowledge of what is seen in the microscope is primarily the result of 
his labours. It is possible that the eye of the chemist may be as capable of 
microscopic culture as that of the geologist, and therefore he himself may be 
- fully qualified to put the proper value on his work ; while it is very probable 
_ indeed that the eye of the mineralogist has even the advantage, in the recogni- 
| tion of thé several components of a minutely crystalline mass. Let us inquire 
_ how far the histological geologist can advance in independent isolation. The 
numerical range of his power of recognition of mineral constituents is very 
limited indeed, and uncertain at the best.. Hornblende can be distinguished 
from augite,—not always ; orthoclase from the plagioclastic felspars always,— 
| but what then? Then the geologist has arrived but at the commencement of 
the examination, which it behoves him to prosecute ; and the microscope is, in 
the present state of our knowledge, powerless to enable him to discriminate. 
between the felspars further. Ae : 
_ His primary objection, moreover, is a hypercriticism formed on quite mis- 
taken grounds. The geologist examines cryptocrystalline rocks, the ingredients 
of which were paragenetic, or nearly so, in time. The chemist seeks out highly 
perfect giant crystals, which have crystallised out of these rocks through exfil- 
tration ; and which are crystals of great size and perfect form, just in virtue of 
their purity. These crystals are associated, it is true, with those of other 
substances; but they are not to any great extent incorporated with one 
another ; just because, though paragenetic in space, they were not so in time. 

