66 ARTICLES REFERRED TO IN THE SECRETARY'S REPORT. 
had the dusty film, like that which accumulates on glass, polished off it a dozen 
times.” 
Besides other interesting optical particulars, the memoir contains many direc- 
tions for the successful practice of celestial photography, some of which might 
be found equally valuable for terrestrial purposes ; and to these we may advert 
on a future occasion, adopting for the present the author’s closing remarks : 
“Tn concluding this account of a silvered glass telescope, I may answer an 
inquiry which, doubtless, will be made by many of my readers, whether this 
kind of reflector can ever rival in size and efficiency such great metallic specula 
as those of Sir W. Herschel, the Earl of Rosse, and Mr. Lassell? My expe- 
rience in the matter, strengthened by the recent successful attempt of M. Fou- 
cault to figure such a surface more than thirty inches in diameter, assures me 
that not only can the four and six feet telescopes of those astronomers be 
equalled, but even excelled. Itis merely an affair of expense and patience. I 
hope that the minute details I have given in this paper may lead some one to 
make the effort.” 
BRIEF REVIEW OF A MEMOIR ON THE CRETACEOUS REPTILES OF THE 
UNITED STATES, PUBLISHED IN THE FOURTEENTH VOLUME OF THE 
SMITHSONIAN CONTRIBUTIONS TO KNOWLEDGE. 
BY THE AUTHOR, JOSEPH LEIDY, M. D 
THE cretaceous formations are so named from the Latin creta, chalk, from the 
fact that the chalk of England forms the most striking rock of the series. 
They underlie or immediately precede the tertiary formations, and contain a 
great many fossils. Plants are rare because the cretaceous rocks are all of 
marine origin, or consist of deposits which were formed at the bottom of the 
ocean. ‘The remains of animals are numerous, but they are confined to the 
lower orders, as no authentic traces of birds or mammals have yet been dis- 
covered in them. 
Cretaceous formations, as indicated by their contained fossils and relative 
position to other rocks, are widely extended throughout the United States, 
though they contain no true chalk. They are mainly composed of beds of 
sand, clay, soft and compact limestones. Among the sandy layers are extensive | 
deposits known as green-sand, which, under the name of marl, is much em- 
ployed as a fertilizing material. 
The cretaceous formations extend in a large tract through New Jersey, 
Maryland, and Delaware, and appear in isolated patches in North and South 
Carolina and Georgia. From the western part of the latter State they curve 
in a wide crescent-like tract through Alabama, Mississippi, and Tennessee, to 
the mouth of the Ohio river. Thence passing in a narrow band through 
Arkansas, they expand so as to occupy a great portion of the region between 
