398 Tlie Aincrican Geologist. June, i898 
testing of building stones little consideration is given to the 
causes intluencing their various properties. In judging the 
resistance, which a stone shows towards weathering, care 
should be taken to recognize the character of the forces to 
which it has been subjected. The strength of a stone bears 
no relation to its mineral components, but is dependent on the 
shape and arrangement of the mineral grains and character 
of the cementing material. In considering the strength of a 
stone four facts have to be kept in mind, viz. : interlockment 
of the particles; coherence, dependent on character of the ce- 
ment and adhesion of the grains; rigidity; and tension. 
The "cjuarry sap," Dr. Julien believes, plays a more im- 
portant role than has hitherto been recognized, as it probably 
carries much of the cement in solution and deposits it only 
when the stone is exposed to the air. This accounts for the 
hardening of the stones after being cjuarried. A distinction 
should also be made between porosity due to cavities between 
the grains and that due to interstices in the individual miner- 
als. The former is a source of weakness, the latter not. al- 
though either may cause the rock to exhibit a high absorp- 
tive capacity. All of these points which have an impnrtar.t 
bearing on the strength of building stones are best studied 
with the microscope. The paper was illustrated by means of 
sections thrown on the screen with a polarizing lantern. Dis- 
cussion was by Prof. Kemp and Mrs. Dudley. 
The second paper of the evening was by J. D. Irving, on 
"Contact-metamorphism of the Palisades Diabase." Mr. 
Irving referred to the work done by Profs. Osann and An- 
drae some years ago, and stated that his results agreed with 
theirs, but recent railroad excavations at Shadyside had en- 
abled him to obtain additional facts. The diabase flo\\' be- 
comes denser, finer grained and porphyritic towards the con- 
tact, with a decrease in hypersthene. It is also conformable 
with the Newark shales. In addition to the zones found by 
Osann, Mr. Irving found: (i) a normal hdrnfels zone rich in 
Spinel; (2) a hornfels zone with brown basaltic hornblende 
layers; (3) hornfels with an undeterminable isotropic mineral 
resembling leucite; (4) hornfels with andalusite. becoming ar- 
kose farther from the contact. This diabase is to be consid- 
ered as an intruded mass and not a surface flow. The paper 
was discussed by Profs. Kemp and Dodge. Dr. Hovt\- and 
Mr. White. 
Heinrich Ries, Secretary. 
