240 The American Geologist. October, 1895 
an interambulacral space. The two boundary rows of an interambulac- 
ral area are derived from the two initial plates of that area which be- 
long to the peristomial ring: while the intermediate meridional rows up 
to the number of six (the total numVjer af rows thus being eight) origi- 
nate in the place and manner here described. 
Prof. Alpheus Hyatt commented on this paper, that it is a very val- 
uable contribution to the life history of the family and the class, and 
also to the general subject of evolution. 
On Asbestos and Ashestiforvi Minerals. George P. Mekrill, Wash- 
ington, D. C (Read by title.) The author treats of the composition, 
mode of occurrence and mineralogical nature of the various minerals 
commercially grouped under the name of asbestos, and attempts to ex- 
plain their fibrous structure as due to abnormal elongation of the min- 
eral parallel to the vertical axis, the individual fibers being in part at 
least bounded by prismatic faces, that is, by the planes of easiest cleav- 
age. The primary cause of this elongation is believed to be mainly dy- 
namical, a result of shearing and other earth movements such as are 
productive of uralitic hornblendes, schistosity, or even platy structure 
and slickensided surfaces where actual fracturing takes place. 
Pre-Camhrian Volcanoes in southern Wisconsin. William H. 
HoBBS, Madison, Wis. A preliminary report on a group of isolated 
areas of igneous rocks which protrude through the Potsdam sandstone 
in the valley of the Fox river, Wisconsin. Some of these areas repre- 
sent local outflows of rhyolitic lava which exhibits superb examples of 
spherulitic, perlitic, fluxion, and breccial structures. The originally 
glassy ground mass of these rocks has become devitrified — hence they 
are apo-rhyolites, and they have been subjected to dynamic metamor- 
phism and subsequent infiltration of silica. They are intruded by dikes 
of both basic and acid rocks. (Specimens and photographs of sections 
were exhibited.) 
A Qeological Sketch of the Sierra Tlayacac, in the State of Morelos, 
Mexico. A. Capen Gill, Ithaca, N. Y. The Sierra Tlayacac, some 
six miles long, situated on the south side of the great fault-line des- 
cribed by Felix and Lenk, consists of a projecting group of mountain 
tops in the midst of the Morelos plain. The plain is formed by the lava 
streams and ejectamenta of Popocatepetl or neighboring volcanic vents. 
The tops of the nearly submerged mountains show that the folding and 
elevation of the Cretaceous (Caprma?) limestone was accompanied or 
followed by the deposition of a limestone conglomerate, in the pebbles 
of which are also Caprina (?) fossils. Absence of pebbles derived from 
eruptive rocks indicates that the volcanic activity of the region was 
subsequent to extensive folding and erosion. 
The limestone agglomerate is overlain by an acid eruptive, and both 
rocks are cut by numerous quartz-pyroxene dikes which show a close 
"consanguinity" with the recent extrusions of Popocatepetl. The very 
striking metamorphism produced by these dikes corroborates the view 
that there is little, if any, migration of material from the intruded mass 
into the metamorphosed rock. 
