744 The American Naturalist. [September, 
granulitized and then drawn out into lenticular areas. The feldspars 
of the gneisses appear to have been recrystallized, since the feldspathic 
areas consist of single feldspar individuals and not fragments of grains. 
The pyroxene also differs from the gabbro pyroxene. It has lost its 
characteristic black inclusions and has assumed a deep green color. 
This mineral, as well as the hornblende, which is abundant in the 
gneisses, are both regarded as having recrystallized, the augite material 
coming from the original augite of the gabbro and the hornblende from 
the secondary amphibole so common in the gabbro. The gneisses are 
thus schistose gabbros in which recrystallization has taken place with 
attendant granulitization. The author points out the fact that in the 
first stages in the alteration of the gabbro scaly hornblende and scapo- 
lite are formed, while in the final stage they have completely disap- 
peared, and in this latter stage there results a gneiss which bears no 
evidence of having been crushed. 
Notes.—The serpentine near Bryn Mawr, Penna., has resulted by 
the alteration of a peridotite according to Miss Bascom.’ The rock of 
the Conshohocken dyke is a typical diabase. 
GEOLOGY AND PALEONTOLOGY. 
Fossil Jelly Fishes.—Certain curious forms, locally known as 
“star cobbles,” have long been found in the middle Cambrian shales 
and limestones of the Coosa Valley, Alabama. They occur at two hori- 
zons associated with silicious concretions. The “star cobbles” are 
recognized by Mr. Walcott as fossil meduse, and among the 8,000 
specimens now in the collections of the U. S. Geological Survey he has 
separated two types allied to the recent Discomeduse. From the large 
number of specimens that have been found over a relatively small 
area, it is evident that they were gregarious and very much like the 
modern Rhizostome (Polyclonia frondosa) in their habits. 
The author describes three species, and refers them to two new genera, 
Brooksella and Laotira, which he also defines. These forms, Brooksella 
alternata, B. confusa and Laotira cambria, together with Dactyloidites 
asteroides, the author groups in the family Brooksellidx, and gives a 
diagnosis of the family. 
8 Proc. Amer. Acad. Science, 1890, p. 220. ° 
