104 Review of Recent Geological Literature. 
in width from two to four feet near the end of the cliflF, the width of 
each cohimn being quite constant throughout its length. The prisms 
have no uniform number of sides, four, five and six being those most 
frequently observed; the sides are unequally developed, but at a dis- 
tance the general effect is quite regular. The author thinks that the 
exceedingly rare occurrence of columnar structure in obsidian is prob- 
ably owing to the fact that the conditions favorable for the production 
of prismatic structure and also for the solidification of the lava as 
amorphous glass are seldom coincident, the cause of columnar struc- 
ture being unquestionably the shrinkage of a homogeneous rock which 
is cooling at a moderate rate from its surface. 
The rock forming the lower part of the columns is dense, black 
obsidian, with thin lithoidal bands or layers of spherulites, which 
range from microscopic size to an inch or more in diameter, having a 
radially fibrous internal structure. Higher up the rock is less massive 
and contains lithopbysfe, as hollow spherulites are called. The lithoidal 
rhyolite into which the obsidian passes northward contains a multi- 
tude of lithophysje of the utmost delicacy and beauty, ranging in size 
up to a foot or more in diameter. Most of them are hemispherical and 
consist of a group of concentric shells which curve one over another 
like the petals of a rose. The partition walls are generally very thin, 
and often close together, in one instance fifty occurring within a radius 
of two inches. They are very fragile and crumble under the touch, 
being made up of small and slightly adhering crystals with brilliant, 
glistening faces. 
Analyses show that the chemical composition of the spherulite is 
essentially the same as that of the surrounding rock, being a small 
portion of the magma which has crystallized with a particular struc- 
ture ; and, further, that the lithophyspe have the same composition as 
the dense spherulites, which shows that the transformation of a spher- 
ulite to a lithophysa can only be a modification of its structure, a 
rearrangement of its minerals, without any chemical addition or loss. 
The conclusion reached by Mr. Iddings, from his very thorough inves- 
tigation, is that the lithophys?e in the obsidian, with their contents of 
prismatic quartz, tridymite, adular-like and tabular soda-orthoclase. 
magnetite and well crystallized fayalite, areof aqueo-igneous-origin and 
result from the action of absorbed vapors upon the molten glass from 
which they were liberated during the process of crystallization con- 
sequent upon cooling, since which time no alteration, chemical or 
mechanical, has taken place. 
Report on the geology of Martha^s Vineyard. By Nathaniel S. 
Shaler. Pages 297-363 ; plates xix-xxix. (Accompanying the seventh 
annual report, U. S. geological survey.) The island of Martha's 
Vineyard belongs to an extensive fringe of low land, mainly composed 
of glacial and modified drift, which borders the continent from New 
York city to cape Cod. The part of this belt of glacial deposits which 
is above water is composed of an outer series of drift ridges which 
