272 The American Geologist. April. i893 
seams. Many of these sections are accompanied by illustrations show- 
ing the relative position and thickness of the different layers and espec- 
ially of the coal beds. 
The first fifty pages contain that which is of most interest to the ge- 
ologist outside of the state. Within these pages are briefly discussed 
the distribution, topography, lithology, stratigraphy, and process of 
deposition of the Coal Measures. Speaking broadly, that part of the 
state lying to the northwest of a line drawn from the extreme north- 
eastern to the southwestern corner of the state is underlain by Coal 
Measure strata. Along this line these strata thin out and disappear, 
but they increase in thickness away from it, reaching a maximum of 
about 2,000 feet at the northwestern corner of the state. The dip is in 
the same direction but is slight, being only about ten feet to the mile. 
The Lower Carboniferous rocks immediately underlie the Coal Measures 
and the two formations are separated, at least at the margin of the lat- 
ter, by an erosion interval. The coal beds are more abundant and 
thickest over the marginal portion of the Coal Measures, but are found 
throughout the interior. 
Under the title, "The process of deposition," a very ingenious and 
sa,tisfactory hypothesis is outlined. Starting from the following con- 
ditions, — that the marginal conditions were generally those favorable 
to the formation of coal beds, that marine and deep water conditions 
were more frequent over the central area and that the strata from the 
base to the top were, at intervals, near the surface, — which are found 
to be necessary for any interpretation of the process of deposition, Mr. 
Winslow clearly presents the succession of events which explain the 
deposition. He begins with discarding the prevalent ideas that the Coal 
Measures are strictly divisible into a lower, middle and upper series, 
that there must have been a subsidence over the entire area of some 
2,000 feet during the time of accumulation, and that a slight tilting and 
an enormous amount of erosion must have taken place to bring the strata 
into their present position. He suggests that when any area is sub- 
merged deposition goes on fastest at the margin, but the deposits here 
are not as thick as those in the interior provided time is allowed for 
all of the submerged area to be filled in. The margin is the first to be- 
come a shallow water area suitable for the accumulation of coal, and as 
the outer portion was gradually filled with sediments the coal swamp 
would slowly creep outwards until it covered the whole in a continuous 
sheet, — provided deposition was continued and subsidence arrested. The 
result is that the coal bed would be thicker near the margin where the 
time of accumulation was longer. A later subsidence of the central 
area, but of small amount, or none at all, at the margin, would make 
the following sediments slightly unconformable upon the earlier ones; 
and when shallow water conditions were reached coal would again form 
and the later coal bed would be nearer the lower one at the margin of 
the submerged area — actually a part of it were thei-e no marginal sub- 
sidence — than towards the interior. In such a manner, with successive 
p'eriods of subsidence and accumulation, — the subsidence always being 
