Human mortality widely differs in rate and seasonal variation from region to region in the world. A study in the seasonal variation of mortality in 18 countries reveals that there are three typical forms of seasonal variation in the 1060's. First, the seasonal variation is getting moderate in the North European countries in high latitudes and with severe cold and in North America (the United States and Canada) with a wide range of temperature changes in the year. Second, deaths still remain concentrated in the cold season in the comparatively warm zones, including Japan, England and the western and the southern parts of the European Continent. Third, what may be called an intermediate type of seasonal variation is seen in such countries as Germany and Switzerland.
It is also to be noted that these three types have been changing chronologically. At present the current tendency of moderation in seasonal variation is getting more conspicuous in the first countries, and that of the winter concentration in the second countries. Even among the countries with the same form of seasonal variation as, for instance, between Japan and England, there are various differences in chronological process and death rate.
These forms of seasonal variation appear to be ascribed, for the most part, to different methods of room heating, or varying types of artificial climate. In North Europe, Canada and the United States, for example, outside temperature drops far below zero in winter, so people cannot live in such environmental conditions without large-scale heating. Naturally, vigorous efforts are made to create and promote artificial climate through central heating, and this appears to have greatly contributed to the steady flattening of the winter death peak, or moderation in seasonal variation.
In such countries as Japan, England and Italy, on the other hand, it is comparatively warm in winter, so people are well able to make living even in the cold season without central heating. In other words, room temperature is usually kept at lower degrees, which appears to have checkmated the otherwise flattening of the winter peak. If central heating had been introduced earlier, the concentration of mortality in winter would have tapered off markedly in these countries as well. After all, it is now essential for these countries to reduce mortality in winter as they have already succeeded in flattening the summer peak.
From the foregoing, it might well be concluded that the steady slowing-down of seasonal variation with the very low death rate as seen in North Europe, Sweden in particular, is an ideal form of human mortality, which is no doubt a final outcome of large-scale central heating requiring huge costs of construction, highly-developed medical techniques, wide medical service, well-planned social security and so forth.
An extensive study by region, climatic and geographical, indicates that the recent moderation of seasonal variation is not at all universal in the United States, but that there are conspicuous differences from region to region. Generally speaking, the seasonal variation is getting particularly moderate in the industrialized regions and much less moderate in the backward states in the South, especially among the Nonwhites with very high infant mortality in winter and the high death rate (they still remain on the stage of winter death concentration). In striking contrast, infant mortality is considerably low in the cold northern regions because babies are protected by comfortable artificial climate.
The winter peak of mortality in the Black Belt is attributed, in final analysis, to the poverty of the Nonwhites (bad diet, unhealthy housing, etc.). Needless to emphasize, the Black Belt widely differs in death rate and seasonal variation from other parts of the United States, but chronological changes have occurred even there: the death rate is getting lower and the summer death rate has nearly disappeared.

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A large scale pyroclastic flow deposit is distributed to the north of the Aira Caldera, South Kyûshû. The deposit, called Ito pyroclastic flow deposit, occupies the area lower than about 400m in altitude surrounded by higher basement mountains.
The deposit attains to 160m or more in maximum thickness, and it looks homogeneous in texture from top to bottom, except the welded zone. Consequently, it is very difficult to separate the deposit into some different flow units.
In the eastern half of the area studied, welding has taken place in the lower part of the deposit (Figs. 3, 4, 7). The welding ranges from incipient to dense. In the welded zone, various features such as color, hardness, texture, etc. change transitionally as exemplified by the systematic vertical variations of dry density (Fig. 7). The features and the pattern of zoning owing to the welding of the deposit indicate that the deposit constitutes a simple cooling unit.
Grain sizes of li.thic fragments contained in the deposit decrease with the increase of the distance away from the Aira Caldera. This fact suggests that the source of the deposit is located there (Fig. 10).
The date of eruption of the Ito pyroclastic flow, determined by the 14C of the carbonized trunk contained in the deposit, is about 16, 000-27, 000 y. B. P.
Although the deposit has been deeply dissected and modified into mesas or buttes (the so-called Shirasu Plateau), some of their surfaces are very flat and even, and thought to be original surfaces. The altitude of the surfaces increases gradually with increasing distances away from the Aira Caldera, i. e., the source of the deposit (Fig. 11). In other words, these surfaces incline gently at an angle of less than about 3 degrees as a whole toward the source. Such a surface of pyroclastic flow deposit inclining toward the source is here named “a reversely inclining surface”. The cause of the reversely inclining surface is discussed below on the basis of topography, general structure of the deposit, general geologic setting, etc.
Firstly, the distribution of the Ito pyroclastic flow deposit is restricted as a whole to the lower area of the basement as mentioned before. Within the lower area, thickness of the deposit before welding as well as that after welding, i. e., that at present, varies in response to the changes in general relief of the basement (Figs. 4, 5, and 6). Thus the area where the deposit is the thickest coincides with the lowest area of the basement. Similarly, not only thickness of the welded zone but also degree of welding of deposit change concordantly in response to the areal variation in thickness of the deposit (Figs. 6 and 8). Moreover, the general configuration of the original surface reflects the underlying topography as a whole (Figs. 5 and 11).
Secondly, welding compaction of the deposit was calculated at several localities, using dry density curves of the deposit, in order to estimate subsidence of the original surface due to welding (Fig. 7). Procedure for calculation of the welding compaction is shown in Fig. 12. The calculation is based upon the assumption that the deposit before welding was homogeneous throughout it in dry density, as inferred from nearly equal dry density of upper and lower nonwelded zone shown in Fig. 7. The restored altitude of the original surfaces before welding in different localities are also shown in Fig. 7. Although the restored contour lines of the original surfaces before welding are somewhat different from those after welding, the surface of the deposit before welding remains reversely inclining (Fig. 13). The deformation (subsidence) of original surfaces due to welding is, therefore, thought qualitatively to be not so great as to vary the original surface from normal inclination to reverse one.

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The writer observed the gentle slopes extending at the foot of Shiragatake block mountain located in the southwestern Tanba Mountainland. This area is composed of the tuffaceous breccia rocks which are acid volcanic similar to liparite rocks. The area is truncated by the tectonic lines in NE-SW direction and S-N direction. The gentle slopes develop along these tectonic lines.
A distinct folding point can be recognized on the slope curve between the steep part of the mountain mass and the gentle part at the foot of the mountain. This longitudinal profile of the slope shows a concave curve as a whole and gentle slopes develop from 200 to 300m. above sea level. The gradient is about 10°. The gentle slopes extend on the valley floor dissecting the mountain mass. They are 200 to 500m. in width and 1000 to 2000m. in length. Several gentle slopes take the form of terrace due to the under cutting by rejuvenation of fluvial erosion.
The gentle slopes spread over the surface which is composed of angular gravels and soil materials produced by deep weathering of the bed rock. The dimensions of those materials are badly sorted. Those materials are 5m. thick on an average and less than 10m. for the maximum. Their colors are partially reddish brown or yellowish brown. The contained gravels are deeply weathered and consolidated very hard with each other.
The surfaces of the gentle slopes are not conformable to the surface of accumulated materials but to the eroded surfaces which were formed on the bed rock by erosion and weathering. For the moment, however, it is not possible to elucidate the characters of agency, process and time of formation of the gentle slopes.

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