Philadelphia Meeting of Geological Society. — ITpham. 93 
• 
Profs. A. C. Lane and C. H. Hitchcock, in discussion, brought oul 
the facts that in the gabbros the change to gneiss is generally marked 
by a passage of pyroxene to hornblende, and that the igneous series, 
though called Upper Laurentian by Prof. Kemp in following the ('ana 
dian usage, is doubtless of later age than the crystalline limestones of 
the region, which would be called Algonkian by many geologists of tin- 
United States. 
The Importance of Volcanic Dusst and Pumice, in Marine Deposita. 
X. S. Shaikh. Cambridge Mass.. Considerations based on the volcanic 
action in the Java district make it probable that the extrusions of rock 
matter in the form of dust and pumice may exceed that which is car 
ried to the sea by the rivers and possibly equals that which is conveyed 
to the ocean by all other actions. Observations on the shore of the 
United States afford evidence that there is a noticeable contribution of 
pumice to the deposits forming along that coast line. The facts warrant 
the supposition that the value of these volcanic contributions to secli 
mentation has not been properly appreciated. 
In the ensuing discussion, Prof. Hitchcock, referring to the discov 
eries of pumice alou# the southern coast line of the United States, re 
marked that in his observations in the West Indies he had found no 
pumiceous rocks among the volcanics. and suggested the possibility of 
remoter sources. 
Dr. C. W. Hayes spoke of the vast formation of vulcanic tuft's seen 
by him in eastern Alaska, extending over many hundred square miles 
ami up to 7."> feet thick. Its bulk he estimated as more than a hundred 
cubic miles, lie also referred to the top layer of the Devonian rocks in 
the southern Appalachians, regarded as a volcanic tuff, which varies 
from 8 to 18 inches in thickness and readies from eastern Tennessee and 
Georgia to Arkansas and Missouri. 
Dr. L. V. I'irsson mentioned the wide area over which the tine dust 
ejected from Krakatoa was spread, and also noted Baekstroni's detection 
of volcanic dust in the sea beaches of Norway. Much of the latter is 
demonstrably from Iceland. Caution is needed against being misled bv 
artificial slags and cinders. 
Dr. M. E. Wadsworth cited the tun's collected by Garman, Merrill. 
and Diller in Nebraska : and Dr. Pebsifob Fbazer recalled the collec 
Hon of dust by Wharton in Philadelphia from the first snowfall of De 
cember, 1883, agreeing entirely in its microscopic characters with sain 
pies from Krakatoa. where the most remarkable volcanic eruption of 
this century had occurred in August of the same year. 
Commenting on the length of time during which volcanic dust may 
remain suspended in the atmosphere, Prof. W. M. Davis stated that 
the peculiarly colored sunsets following the Krakatoa outbreak lasted 
through 1884 and that the so-called Bishop's ring was visible around 
the sun for fully two years. 
Prof. ShaLEK mentioned the observations in Germany, alter the 
Krakatoa eruption, of shining clouds at first having an elevation of su 
miles in the air and later rising to no mill's before they disappeared. 
