618 THE JOURNAL OF GEOLOGY. 
necessary to demonstrate their falsity before they could be laid 
aside. Prominent among them was the. belief, that the age of 
a rock can be determined by its lithological characters. In the 
absence of fossils, the granular quartz was thus determined to be 
metamorphozed Potsdam Sandstone, and both Cambrian and 
Hudson River Slates were included under the name Taconic 
Slates, and supposed to be of pre-Cambrian age. A second error 
was the belief that the presence of certain minerals may be 
relied upon for identifying horizons—and a third point, which 
though known to be false, was too much trusted in even by those 
who knew better; viz., that actual succession of rocks in a meta- 
morphozed region is a safe-guide in determining the order of 
sequence. In 1871 Professor Dana entered the field determined 
to settle the disputed questions by study of the region itself. 
‘‘My purpose,” he wrote, ‘‘was (1) to prove the continuity from 
north to south of the three associated Taconic formations, the 
quartzite, the limestone and the slates or schists; also (2) to 
work out the system of flexures; (3) to ascertain whether the 
Taconic mountains were generally or not of synclinal structure, 
as they were made by Rogers, Mather and Hall, and in 1864 by 
Logan; (4) to settle the question as to continuity from east to 
west of the limestones of the different north-and-south belts; 
{5) to apply the evidence from fossils, making them the sole 
basis for fixing the age of the beds; and finally (6) to use the 
evidence of the age, thus obtained, for the determination of the 
age of the hydromica schists, chloritic schists, garnetiferous and 
staurolitic schists, and other rocks of the Taconic Mountains, 
and thus test the value of, or give greater precision to, the 
assumed ‘lithological canon’ first propounded by Emmons. 
My work was continued in western New England and eastern 
New York at intervals from 1871 to the close of the season of 
1886.” The results of these investigations are continued in a 
number of papers in the American Journal of Science, from 1871 
to 1888. Among the more important of them are the following, 
viz.: ‘‘What is true Taconic?”’ ‘Green Mountain Geology: on 
the quartzite” in 1872; ‘‘On the Quartzite, Limestone and asso- 
