404 PROP. J. w. jrDD o^" the tertiaet assb 



It will be observed that tbis analysis differs from that of biotite, 

 the only uniaxial mineral which suggests itself for comparison, by 

 the low proportion of ferric oxide (which is only to some extent 

 compensated for by the higher alumina-percentage), by the large 

 proportion of lime and water, and the probably low percentage of 

 tbe alkalies. 



Under these circumstances it became necessary to make as accurate 

 an examination of the optical properties of the mineral as was 

 possible, and in doing this I had the great advantage of the advice 

 and assistance of Mr. L, Fletcher. Elakes of the mineral were 

 mounted in Canada balsam, and examined by means of a microscope 

 constructed by Xachet, of Paris, on the pattern described by 

 MIT. Fouque and Michel Levy*. This microscope has the great 

 advantage of permitting the interference-figures, exhibited by 

 the polariscope with the converging system of lenses of Bertrand 

 and von Lasaulx, to be examined even with the highest powers. 

 MVe found in this way that not only was the mineral practically 

 uniaxial, but that even when compared with other so-called uniaxial 

 crystals, like the Yes avian meroxene, for example, the interference- 

 figures indicate an excessively small axial angle. The only mica 

 with anything like so small an axial angle which we were able to 

 find was a remarkable pale-coloured, silvery biotite from Easton, in 

 Pennsylvania, which does not appear to have been analyzed, though 

 its optical characters have been described by Blakef. 



In the determination of the other minerals in the rock, far less 

 difficulty was experienced. 



The most abundant constituent of the rock, as seen in thin sections 

 under the microscope, was found to be a green hornblende. In the 

 greater part of the rock the characters of this mineral were clear 

 and unmistakable. By transmitted light its colour is a very pale 

 green, and its pleochroism, though distinct, was by no means vivid. 

 Basal sections afforded the means of measuring the angle of the 

 principal cleavages as exhibited in well-marked cleavage cracks, and 

 left no doubt as to the species of the mineral, and this is confirmed 

 by its extinction- angle. In many places the hornblende is very 

 clear and fresh-looking, and free from inclusions of all kinds except 

 the grains of magnetite scattered through it. But in places this 

 pale green mineral exhibits traces of the peculiar tabular inclusioiis 

 of diallage, and in such portions of the crystals the extinction-angle 

 indicates that we are dealing with an augite rather than with a 

 hornblende. The conclusion to which I am led by the study of a 

 large series of preparations of this rock is, that the mineral was 

 originally augite, that it was by Schillerization converted into diallage, 

 and that subsequently this diallage was amphibolized. In many 

 slices of the rock, however, the mineral is a perfectly clear and 

 fresh-looking hornblende, and exhibits no trace of its secondary 

 origin. 



Enclosed in the hornblende, and often penetrating into its fissures, 



* Mineialogie Micrograpliique (1879). p. 27. 

 t Amer. Journ. Sci. vol. xii. (1851) p. (5. 



