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GEOLOGY OF THE FOURTH DISTRICT. 
CHAPTER II. 
General Features of the Strata , Topography , etc. 
In describing the rocks of the Fourth Geological District, according to their physical or 
lithological characters, and the effects of these in modifying the contour of the surface, we 
shall find the whole area occupied by several successive parallel groups or associations of 
strata, each possessing characters which distinguish them from others above and below. The 
general line of their bearing or strike is nearly east and west, with some slight variations 
owing to denudations, but never to uplifting or derangement. This will be clearly seen on 
reference to the colored geological map. Throughout the whole distance, there are no dis¬ 
turbances of much importance ; and the greatest effects produced in any case, are slight dis¬ 
locations extending for a few yards ; or sometimes gentle undulations of the strata, which may 
affect them for several miles. To this character of the country we owe its great simplicity 
of structure, the order of succession being scarcely any where obscured except by superficial 
detritus. This in many places, and for considerable extent, covers the nearly horizontal 
strata, and in some instances renders the local succession obscure. Still the numerous river 
channels and ravines, the deep excavations of the north and south lakes, all aid in developing 
in the most satisfactory manner the whole series from the highest to the lowest rock. And 
although we have not to describe the great changes and important modifications wrought 
upon the strata by plutonic agency, yet we have exhibited in the most perfect manner the 
natural arrangement of the deposits, the situation and condition in which they were left by 
the agents of their production ; and we can recognize, unchanged by subsequent influences, 
the nature of deposits at remote points from each other. We trace a rock through all its 
grades of coarser materials to finer and finer, until at last we find it composed of comminuted 
matter which slowly sank to the bottom in the deeper parts of the ocean. 
The analogy to recent formations is thus more fully seen; for we have precisely the same 
materials, differing only in degree of induration. We have the unaltered monuments of a 
wide spread ocean teeming with life, and we find recorded its changes through vast periods 
of time. We now learn what were the conditions of its bed at these successive periods, and 
also what different characters it presented at distant points. The varying forms of its inha¬ 
bitants are as well marked and as perfectly preserved, as the recent species amid the mud 
and sand and pebbly bottoms of our present seas. The geographical limits of certain genera 
