14 PREFACE. 
present knowledge, to determine the precise areal distribution of 
the several formations, although the facts as to the structure and the 
stratigraphic sequence of the rocks are given whenever they are con- 
clusive. More detailed study of the region must be made before the 
areal geology can be mapped with any degree of precision. 
The map (PI. IV) prepared by Dr. Gregory indicates the general areal 
distribution of the clastic rocks in Aroostook County, so far as we have 
observed them. The belts of similar rock have a general northeast- 
southwest trend. The Aroostook limestone is on the east, the arena- 
ceous limestones and shales are next westward, and the more sandy 
Sheridan sandstones are in the western part of the area; but this does 
not accurately express the distribution of the formations, for in the 
region marked as sandstone are found not only the Ashland lime- 
stone areas, but slates that are undoubtedly older than the Sheridan 
sandstone, which contains pebbles of apparently these same slate rocks. 
Because of the uncertainty regarding the true sequence of the clastic 
formations, Dr. Gregory has classified them (in Part II) purely on 
petrographic characters. This gives the facts which will make their 
areal mapping possible, but leaves their classification on a stratigraphic 
basis to be completed when further evidence regarding the sequence 
has been obtained. 
The description of the physical and lithologic characters of the rocks 
and geologic formations will be found in Part II. A list of all the 
localities in Aroostook County examined by Dr. Gregory and myself 
is given in Part III. 
The collections here reported upon were made by Gilbert Van 
Ingen, Olaf O. Nylander, Herbert E. Gregory, and myself, and are now 
the property of the United States Geological Survey. We are indebted 
to Mr. Nylander for pointing out numerous localities along the roads, 
in out-of-way places, where he had discovered outcrops, and we 
are particularly indebted to him for knowledge of several important 
exposures containing fossils. Mr. Nylander is a keen observer, and 
deserves great credit for the discovery of fossil localities which it 
would have taken much time and labor to locate without his prelim- 
inary work. 
For many courtesies we are indebted to the officials of the Bangor and 
Aroostook Railroad, and the State land office, and to numerous land- 
owners whose farms we invaded in the course of our surveys, and who, 
in many cases, took considerable pains to promote our investigation. 
H. S. W. 
