2 Geology of Massachusetts. 
er, as they are in nature, in a few general divisions; the rocks in 
each division being so intimately related, that in an economical point 
of view, they may be regarded as varieties ; although, in a scientific 
point of view, their differences are very important. All the rocks 
of a group have acommon color on the map; and the different sorts 
are delineated by means of dots, crosses, circles, &c. In short, it 
has been a great object with me, so to simplify the map as to ren- 
der it easily intelligible; while it exhibits all that is important to 
the practical man, as well as to the scientific enquirer. In the first 
part of my Report, I shall explain the different formations on the 
map, only so far as shall be necessary in illustrating our geology with 
reference to the useful arts; reserving the most important scientific 
remarks to a subsequent period. 
It will be seen that I have extended the map a short distance into 
the adjoining states. This was done chiefly with a view to exhibit 
certain beds of ore, or other interesting minerals, which occur just 
beyond our limits. Jn a statistical point of view, these are nearly as 
important as those found within the State; and for this reason I shall 
notice such minerals in my Report. 
In laying down the geology of the eastern part of Rhode Island, 
I have been much assisted by the communications of Col. Joseph G. 
Totten, of Newport. Iam not without an apprehension that the re- 
gion around Providence, particularly in Cumberland, will be found 
exhibited on the map somewhat incorrectly ; as I had not time, when 
passing over it, to unravel entirely to my satisfaction, the peculiar in- 
tricacy of its geology. In laying down the geology of Berkshire, I 
have been greatly aided by the geological map of that county, pub- 
lished a few years since by Professor Dewey. 
It has been my intention to give to each rock, precisely that rela- 
tive extent on the map, which it occupies on the earth’s surface. ‘To 
do this with perfect accuracy, over an extent of more than seven 
thousand square miles, would be an almost endless task: especially 
when we recollect, that over the greater part of the surface, the 
rocks are covered by loose soil; so that in some instances, no rock 
in place shows itself to the traveller, for an extent of thirty or forty 
miles. In such cases, indeed, this stratum of sand, clay, and gravel, 
has been exhibited on the map under the name of diluvium. Sul, 
under the most favorable circumstances for observation, the effort to 
give on a map the exact boundaries of each particular rock, must be 
regarded as only an approximation to the truth. Yet for all practical 
purposes, such approximation answers nearly as well as entire accu- 
