CONFIGURATION OF THE ROCK FIOOR OF GREATER 
NEW YORK. 
By William H. Hobbs. 
PART I. -STRUCTURAL STUDIES. 
INTRODUCTION. 
AIDS TO GEOLOGICAL STUDIES ON MANHATTAN ISLAND. 
Accurate maps. — The early importance of Manhattan Island in com- 
mercial and economic lines and its entry upon great engineering enter- 
prises which require a penetration of the rock floor have greatly 
facilitated the work of the structural geologist within the area. As 
early as 1821 the city was laid out, and the gridiron of numbered 
streets and avenues has not only been responsible for a precision and 
accuracy of the early maps, rarely obtainable elsewhere, but has given 
to even casual geological observations unusual value. KandalFs map 
of the island, submitted in sections between the years 1811 and 1821, 
now preserved in the office of the commissioner of public works, gives 
the precise locations of all the old farms in relation to watercourses 
and topography. It comprises four volumes of 92 sheets of map, size 
25 by 37 inches, on scale of 100 feet to the inch. General Viele's map, 
on the scale of 1,000 feet to the inch, was published in 1*74," and is 
based upon Randall's map. It shows the original shore line of the 
island, the mainland, the drainage system, the topography, and the 
location of each exposure south of Manhattanville, all superimposed 
upon the gridiron of streets and avenues. The accuracy of this ma}) 
has been abundantly attested by engineers, real estate men, and others, 
who habitually use it, and, so far as the exposures of rock are con- 
cerned, it has been tested by the writer, both by comparison with the 
early reports of geologists upon Manhattan Island and by examination 
in the field. 
Sections along the cross streets of the city were early made to estab- 
lish the grade, and many of the engineers made careful note in theii 
aViele, Egbert L., Topographic Atlas of the City of New York. 1874. 
