260 C. H. SMYTH, JR.—WEATHERING OF ALNOITE. 
about the same degree as the upper walls of the gorge. To what extent 
the recession of the dike is due to natural causes and to what extent 
resulting from the quarrying operations it is impossible to say. ‘The 
weathering of the walls can not be taken as indicating an early removal 
of the dike, as the weathering of both rocks would go on together, and 
excavation would doubtless show weathered walls extending as far as 
weathered dike-rock. 
Though sufficiently coherent to stand with an overhanging face and 
to furnish by careful digging lumps several inches in diameter, the 
weathered rock has little of the toughness of a true clay, being readily 
erumbled in the hand, even while moist. Both in consistency and in 
_size of grain the material is about midway between clay and sand, though 
if either term were to be applied to it the former rather than the latter 
would be used. Such material would fall an easy prey to the agents of 
erosion did not its occurrence as a dike prevent its removal. This mode 
of occurrence, moreover, has not only preserved the weathered rock for 
study, but has kept it in a state of almost ideal purity. Indeed, there 
seems to be but one way in which any superficial foreign mineral matter 
could find its way into the clay. The dike to the very bottom of the 
exposure is penetrated by abundant roots, sometimes as much as half an 
inch in diameter, and it is possible that on their death and decay chan- 
nels might be formed through which soil would find its way into the 
dike. But as the soil immediately above the outcrop is hardly a foot 
thick, and is probably largely derived from the dike itself, the amount 
of impurity from this source can hardly be worthy of consideration. 
It is entirely possible that mineral matter has been brought in from 
the wall rocks, but if so it would doubtless be as carbonates (and it is 
probable that the “ satin spar ” mentioned above is in part of this origin) 
and would not materially affect conclusions based upon the study of 
the rock. Thus the rock occurs in such a way as to fit it particularly. 
well for an investigation of the effects of weathering upon it. 
PbrROGRAPHY OF THE Rock 
In the paper above referred to the petrographic character of the large 
dike has been described, and only a brief review is needed here. 
The fresh rock consists largely of biotite and serpentine derived from 
original olivine, the latter mineral itself being extremely rare. The 
minor constituents are magnetite, apatite, and perofskite, together with 
secondary calcite. No melilite has been aetually determined in the 
large dike; but asin all other respects the mineralogical composition is 
identical with that of the two other dikes, which are rich in melilite, it 
