170 «FD. ADAMS—NODULAR GRANITE FROM PINE LAKE, ONTARIO. 
the rocks. By neglecting, however, those constituents which do occur 
in very subordinate amount and having in view a feldspar in which the 
alkalies are present in the proportion indicated by the analysis, a rough 
calculation shows that the granite is composed approximately of 42 per 
cent of quartz and 58 per cent of feldspar. 
Similarly, in the case of the nodule, if all the alkalies are calculated 
as feldspar and the excess of alumina is calculated as sillimanite, the 
percentage composition would be approximately : quartz, 68 per cent; 
feldspar, 15 per cent; sillimanite, 17 per cent. If muscovite be present 
this will alter the relative proportions of these constituents, although not 
ereatly, as the muscovite present cannot amount to more than a few per 
cent. 
ORIGIN OF THE NODULAR STRUCTURE 
Granites and allied rocks containing spheroidal or concretionary lumps 
or nodules are known from many parts of the world and some of these 
occurrences are widely celebrated, as, for instance, the concretionary 
granite from Fonni in Sardinia, the “ pudding granite” of Vermont, and 
the orbicular diorite of Corsica. 
The origin of these structures is not, however, in all cases thoroughly 
understood; but in a recent and elaborate memoir* on the subject von 
Chrustschoff has presented the results of a-very detailed comparative 
study of a large number of such occurrences and believes them to be 
genetically divisible into four groups. 
1. Concentric, spheroidal, and concretionary growths about foreign 
inclusions. 
2. Nodular growths about fragments of secretions or inclusions, which 
latter are often partially or wholly redissolved. 
3. Group of the so-called pudding granites, where the structure is due 
to a simple concretionary action set up in the magma during its normal 
crystallization. 
4, Primary structural forms of the magma or endomorphic contact 
products. 
In the Pine Lake occurrence we evidently have to do with the case of 
primary magmatic differentiation, for although in the case of the occa- 
sional vein-like forms the mode of occurrence is such as to suggest a de- 
velopment subsequent to the crystallization of the granite, the fact that 
these pass into spherical nodules precisely identical with those which 
occur scattered as isolated individuals through the rock far and wide, 
and which are far more abundant than the streaked or vein-like forms, | 
proves that they are both identical in origin and are derived from the 
crystallization of a magma which was free to gather itself into rounded 
* Memoires de l’Academie Impériale des Sciences de Saint Pétersbourg,vii série, tome xlii, no. 3, 
1894. 
