PETROGRAPHY OF THE ALNOITE. 261 
seems highly probable that melilite was originally present and has be- 
come obscured in the process of alteration. For this reason the rock is 
classed as alnoite, although the chemical composition, as shown below, 
is more like that of a peridotite; butas this is equally true of the alnoite 
at the fault,* the evidence is of little moment. 
From the foregoing brief statement it is evident that the dike rock is 
far from being in its original condition ; but, it is important to note, the 
changes which it has undergone are of the kind included under the head 
of alteration, the product of agents working at a considerable depth. 
Between such alteration and the more superficial process of weathering 
it is necessary to make a distinction, as Dr Merrill + has accentuated, 
although a sharp line of demarkation can seldom be drawn. 
Samples of the weathered rock show under the microscope a very great 
preponderance of the mica. It has been somewhat bleached and irregu- 
larly stained with iron, but shows as a rule the normal optical prop- 
erties in spite of the partial decomposition which it has manifestly un- 
dergone. The large scales are often pearly or iridescent, very soft and 
inelastic, sometimes closely resembling talc. Magnetite and perofskite 
are abundant, while apatite is probably present. A few very small erys- 
tals have the optical properties of pyroxene, but their exact determina- 
tion is a matter of difficulty. The serpentine of the original rock is no 
longer perceptible and every trace of the calcite has disappeared. In 
addition to the determinable minerals, there is some very fine material 
of uncertain nature, but it is present only in small amount and is a less 
important factor than in most cases of extensive weathering. 
SAMPLES FOR ANALYSIS 
To determine the nature of the chemical changes involved in the 
weathering, both the fresh and the weathered rock were analyzed, with 
the results shown below. Of the fresh rock, a sample was selected that 
showed no calcite veins, although in thin section considerable inter- 
stitial calcite was evident, as is the case in every section of the rock 
examined. 
The sample of weathered rock was taken at a depth of four feet from 
the surface of the ground, being three feet down in the fissure, measur- 
ing from the upper surface of the wall rock. This sample, while suffi- 
ciently near the surface to show nearly or quite the full extent to which 
the weathering has proceeded, was so protected within the walls as to 
be kept from any notable amount of foreign admixture, aside from 
* Cf. Am. Jour. Sei. (3), vol. xlili, p. 325. 
7A treatise on rocks, rock-weathering, and soils, p. 174. 
